Does performance in selection processes predict performance as a dental student?

OBJECTIVE: This study investigated associations between the performance of dental students in each of the three components of the selection procedure [academic average, Undergraduate Medicine and Health Sciences Admission Test (UMAT) and structured interview], socio-demographic characteristics and their academic success in an undergraduate dental surgery programme. MATERIALS AND METHODS: Longitudinal review of admissions data relating to students entering dental education at the University of Otago, New Zealand, between 2004 and 2009 was compared with academic performance throughout the dental programme. RESULTS AND DISCUSSION: After controlling for variables, pre-admission academic average, UMAT scores and interview performance did not predict performance as a dental student. Class place in second year, however, was a strong predictor of class place in final year. Multivariate analysis demonstrated that the best predictors of higher class placement in the final year were New Zealand European ethnicity and domestic (rather than international) student status. Other socio-demographic characteristics were not associated with performance. These interim findings provide a sound base for the ongoing study. CONCLUSION: The study found important socio-demographic differences in pre-admission test scores, but those scores did not predict performance in the dental programme, whether measured in second year or in final year.

Reading embossed capital letters: an fMRI study in blind and sighted individuals.

Reading Braille activates visual cortex in blind people [Burton et al., J Neurophysiol 2002;87:589-611; Sadato et al., Nature 1996;380:526-528; Sadato et al., Brain 1998;121:1213-1229]. Because learning Braille requires extensive training, we had sighted and blind people read raised block capital letters to determine whether all groups engage visual cortex similarly when reading by touch. Letters were passively rubbed across the right index finger at 30 mm/s using an MR-compatible drum stimulator. Age-matched sighted, early blind (lost sight 0-5 years), and late blind (lost sight >5.5 years) volunteers performed three tasks: stating an identified letter, stating a verb containing an identified letter, and feeling a moving smooth surface. Responses were voiced immediately after the drum stopped moving across the fingertip. All groups showed increased activity in visual areas V1 and V2 during both letter identification tasks. Blind compared to sighted participants showed greater activation increases predominantly in the parafoveal-peripheral portions of visuotopic areas and posterior parts of BA 20 and 37. Sighted participants showed suppressed activity in most of the same areas except for small positive responses bilaterally in V1, left V5/MT+, and bilaterally in BA 37/20. Blind individuals showed suppression of the language areas in the frontal cortex, while sighted individuals showed slight positive responses. Early blind showed a more extensive distribution of activity in superior temporal sulcal multisensory areas. These results show cross-modal reorganization of visual cortex and altered response dynamics in nonvisual areas that plausibly reflect mechanisms for adaptive plasticity in blindness.

Neural correlates for roughness choice in monkey second somatosensory cortex (SII).

This experiment explored the relationship between neural firing patterns in second somatosensory cortex (SII) and decisions about roughness of tactile gratings. Neural and behavioral data were acquired while monkeys made dichotomous roughness classifications of pairs of gratings that differed in groove width (1.07 vs. 1.90 and 1.42 vs. 2.53 mm). A computer-controlled device delivered the gratings to a single immobilized finger pad. In one set of experiments, three levels of contact force (30, 60, and 90 g) were assigned to these gratings at random. In another set of experiments, three levels of scanning speed (40, 80, and 120 mm/s) were assigned to these gratings at random. Groove width was the intended variable for roughness. Force variation disrupted the monkeys' groove-width (roughness) classifications more than did speed variation. A sample of 32 SII cells showed correlated changes in firing (positive or negative effects of both variables) when groove width and force increased. While these cells were recorded, the monkeys made roughness classification errors, confusing wide groove-width gratings at low force with narrow groove-width gratings at high force. Three-dimensional plots show how some combinations of groove width and force perturbed the monkeys' trial-wise classifications of grating roughness. Psychometric functions show that errors occurred when firing rates failed to distinguish gratings. A possible interpretation is that when asked to classify grating roughness, the monkeys based classifications on the firing rates of a subset of roughness-sensitive cells in SII. Results support human psychophysical data and extend the roughness range of a model of the effects of groove width and force on roughness. One monkey's SII neural sample (21 cells) showed significant correlation between firing rate response functions for groove width and speed (both correlations either positive or negative). Only that monkey showed a statistically significant interaction between groove width and speed on roughness classification performance. This additional finding adds weight to the argument that SII cell firing rates influenced monkey roughness classifications.

Tactile-spatial and cross-modal attention effects in the primary somatosensory cortical areas 3b and 1-2 of rhesus monkeys.

Neuronal responses in somatosensory cortical areas 3b and 1-2 (S1) were recorded during an attention task involving cue directed selection of one of three simultaneous stimuli: dual sinewave shaped vibrotactile stimuli applied to mirror sites on both hands or a similarly timed auditory tone. The cued stimulus occurred with one of two equally probable patterns: a constant amplitude vibration or the latter with a superimposed brief sinewave amplitude pulse midway during stimulation. Uncued stimuli always contained amplitude pulses. Two monkeys signaled the absence or presence of an amplitude pulse by appropriately moving a foot pedal up or down. Cues initiated trials by marking the location where the monkey had to discriminate the stimulus pattern. Cue location and stimulus pattern varied randomly per trial. Approximately 50% of cells (44/77 in 3b and 39/77 in 1-2) had significantly different firing rates to stimulation cued to the contralateral hand relative to spatially cuing the ipsilateral hand or cross-modally the auditory stimulus. Relatively suppressed firing rates during times prior to the epoch containing amplitude pulses improved signal-to-noise ratios for responses to amplitude pulses. Instances of significant enhanced activity during and after intervals with amplitude pulses were rare and relative to suppressed activity when cues directed attention to the ipsilateral hand or auditory stimulus. The present findings suggest that attention influences even the earliest stage somatosensory cortical processing. Findings were more modest in S1 than those previously seen in S2 (Burton et al., Somatosens Mot Res 14: 237-267, 1997), which supports the concept of multistage attention processes for touch.

Response patterns in second somatosensory cortex (SII) of awake monkeys to passively applied tactile gratings.

This experiment explored the effects of controlled manipulations of three parameters of tactile gratings, groove width (1.07-2.53 mm), contact force (30-90 g), and scanning speed (40-120 mm/s), on the responses of cells in second somatosensory cortex (SII) of awake monkeys that were performing a groove-width classification task with passively presented stimuli. A previous experiment involving an active touch paradigm demonstrated that macaque SII cells code groove-width and hand-movement parameters in their average firing rates. The present study used a passive-touch protocol to remove somatosensory activation related to hand movements that accompany haptic exploration of surfaces. Monkeys maintained a constant hand position while a robotic device delivered stimulation with tactile gratings to a single stabilized finger pad. Single-unit recordings isolated 216 neurons that were retrospectively assigned to SII on histological criteria. Firing patterns for 86 of these SII cells were characterized in detail, while monkeys classified gratings as rough (1.90 and 2.53 mm groove widths) or smooth (1.07 and 1.42 mm groove widths), with trial-wise random, parametric manipulation of force or speed; the monkeys compared 1.07 versus 1.90 mm and 1.42 versus 2.53 mm in alternating blocks of trials. We studied 33 cells with systematic variation of groove width and force, 49 with groove width and speed, and four with all three variables. Sixty-three cells were sensitive to groove width, 43 to force (effects of random force in speed experiments contributed to N), and 34 to speed. Relatively equal numbers of cells changed mean firing rates as positive or negative functions of increasing groove width, force, and/or speed. Cells typically changed mean firing rates for two or three of the independent variables. Effects of groove width, force, and speed were additive or interactive. The variety of response functions was similar to that found in a prior study of primary somatosensory cortex (SI) that used passive touch. The SII sample population showed correlated changes (both positive and negative) in firing rates with increasing groove width and force and to a lesser degree, with increasing groove width and speed. This correlation is consistent with human psychophysical studies that found increasing groove width and force increase perceived roughness magnitude, and it strengthens the argument for SII's direct involvement in roughness perception.

Effects on discrimination performance of selective attention to tactile features.

This study examined selective attention to tactile dimensions by combining a selective cueing paradigm with a test of integrality. In Experiment 1, subjects selectively attended to changes in the frequency or duration of pairs of vibrotactile stimuli and identified the higher frequency or longer duration stimulus. In Experiment 2, using surface gratings in an identical experimental procedure, subjects identified the rougher or longer duration stimulus. In both experiments, greater performance accuracy was found on trials where the cue correctly (valid) predicted the changing dimension, vs incorrectly (invalid) cued or no-cue (neutral) trials. More errors on the invalidly vs neutrally cued trials show the cost of focal attention. Increases in performance on validly vs neutrally cued trials show a benefit of filtering irrelevant stimuli in the cued conditions. Results effectively demonstrate focal attention to tactile features. Tests of integrality, in terms of the effects of correlated change in both dimensions, showed no redundancy gain for either vibrotactile or grating tasks, suggesting that frequency and roughness are separable from stimulus duration. Interference of negative correlated change for frequency but not roughness discriminations may be explained by differences in task difficulty.

Attending to and remembering tactile stimuli: a review of brain imaging data and single-neuron responses.

Clinical and neuroimaging observations of the cortical network implicated in tactile attention have identified foci in parietal somatosensory, posterior parietal, and superior frontal locations. Tasks involving intentional hand-arm movements activate similar or nearby parietal and frontal foci. Visual spatial attention tasks and deliberate visuomotor behavior also activate overlapping posterior parietal and frontal foci. Studies in the visual and somatosensory systems thus support a proposal that attention to the spatial location of an object engages cortical regions responsible for the same coordinate referents used for guiding purposeful motor behavior. Tactile attention also biases processing in the somatosensory cortex through amplification of responses to relevant features of selected stimuli. Psychophysical studies demonstrate retention gradients for tactile stimuli like those reported for visual and auditory stimuli, and suggest analogous neural mechanisms for working memory across modalities. Neuroimaging studies in humans using memory tasks, and anatomic studies in monkeys support the idea that tactile information relayed from the somatosensory cortex is directed ventrally through the insula to the frontal cortex for short-term retention and to structures of the medial temporal lobe for long-term encoding. At the level of single neurons, tactile (such as visual and auditory) short-term memory appears as a persistent response during delay intervals between sampled stimuli.

Tactile attention tasks enhance activation in somatosensory regions of parietal cortex: a positron emission tomography study.

We used positron emission tomography to study cortical regions mediating tactile attention. Cues selectively directed subjects to attend to the roughness or duration of contact with embossed gratings that rubbed against a single fingertip with controlled speed and force. The task required discriminating between paired gratings that differed in tactile features of roughness and/or length. For different blocks of trials, cues directed attention to one tactile feature or indicated a divided attention strategy to a change in either feature. All attention conditions unambiguously activated several somatosensory foci in the parietal cortex, including somatotopically appropriate portions of the primary somatosensory cortex in the postcentral gyrus (S1) and the secondary somatosensory region (S2) within parietal opercular regions. There was no evidence for separate processing foci for selective and divided attention strategies, or for selectively attending to roughness versus stimulus duration. We observed a greater magnitude blood flow change in S2 versus S1 during attention tasks, which suggests that S2 might actually influence S1 activity. Despite these differences, modulation of S1 and S2 supports concepts of early selection in tactile attention. There were also examples of non-sensory foci in frontal cortex, anterior cingulate gyrus and bilateral superior parietal regions at the fundus of the postcentral sulcus. Posterior parietal regions observed in this study did not overlap foci seen in studies of visual attention. Thus, the posterior parietal region may be subdivided into modality-specific subregions, each of which processes information needed to attend to a specific modality. These non-sensory areas may constitute a network that provides a source of modulating influences on the earlier stage, sensory areas.

Vibrotactile stimulus order effects in somatosensory cortical areas of rhesus monkeys.

This study demonstrated effects of stimulus order on single-cell responses in the macaque primary and secondary somatosensory and 7b cortical areas. As part of a study of tactile attention, two monkeys (Macaca mulatta) received similar constant amplitude, sinusoidal tactile vibration patterns (125 Hz) at two glabrous skin, hand locations. The stimuli started asynchronously with offsets of 150 or 300 ms. In cells with bilateral receptive fields and increased firing to a stimulus, we observed an average lowering of 30% in the firing rates to the contralateral stimulus when preceded by stimulation of the ipsilateral hand. Some cells with only contralateral receptive fields showed similar depressed responses to a contralateral stimulus when preceded by an ipsilateral stimulus. Stimulus order effects were more prominent during dual stimulation of the receptive field on one hand. In six cells whose background activity was inhibited by the first stimulus, higher rates appeared at the onset of the second stimulus. These results suggest a possible substrate for psychophysical findings of stimulus masking in which a preceding stimulus depresses detection thresholds. The spatial and temporal characteristics of in-field inhibitory mechanisms best account for the observed stimulus order effects.

Tactile-spatial and cross-modal attention effects in the second somatosensory and 7b cortical areas of rhesus monkeys.

This study analyzed neuronal responses in the second somatosensory (SII) and 7b cortical areas during a selective attention task. Cues directed attention to one of three simultaneous stimuli: vibrotactile stimuli applied to mirror sites on both hands or to a similarly timed auditory tone. Two stimulus patterns appeared with equal probability for the cued stimulus: a constant amplitude sinewave or the latter with a superimposed brief amplitude pulse midway in the trial. Uncued stimuli always contained amplitude pulses. Monkeys demonstrated whether an amplitude pulse at the cued location was present or absent by making appropriately rewarded up and down foot pedal movements. Cue location and stimulus pattern varied trial-wise and pseudo-randomly. Average firing rates to vibrotactile stimuli in 82 of 181 SII cells and 13 of 22 area 7b cells differed significantly during at least one epoch for trials cued to the contralateral hand when compared to trials cued to the ipsilateral hand or auditory stimulus. Predominant were relatively suppressed firing rates during times prior to the epoch containing the amplitude pulses or enhanced activity during and after these pulses. Generally, different cells showed suppression early vs enhancement later in a trial. Analyses of the ratio between firing rates before and during the amplitude pulses suggested improved evoked signals to the amplitude pulses. The discussion considers attention as a mechanism for reducing distractions, early in the trial through suppressing these signals, or for selectively increasing response magnitudes in the cued channel, especially around times when amplitude pulses were present or absent.

Discrimination of vibrotactile frequencies in a delayed pair comparison task.

This study quantified human short-term-memory decay functions for delayed vibrotactile frequency discriminations. Subjects indicated which of two successive intervals contained the higher or lower frequency of a pair separated by delay periods of 0.5-30 sec. Performance decreased as a function of length of delay and was higher when delays were unfilled than when they were filled with a backwards-counting task. This interpolated task may have interfered with rehearsal of a coded representation of the remembered vibrotactile frequency. A change in decay rate after 5-sec delays suggests a switch from reliance on sensory memory to the coded frequency representation. Performance and decay rate depended on presentation order of higher or lower frequency within pairs. Reciprocal performance asymmetries seen in high-versus low-frequency ranges did not result from simple response bias.

Responses in primary somatosensory cortex of rhesus monkey to controlled application of embossed grating and bar patterns.

Responses of 66 neurons in primary somatosensory cortex (SI) of three anesthetized monkeys (Macaca mulatta) were characterized with grating patterns of 550- to 2900-mm groove width (Gw) and 250-mm ridge width, and/or pairs of 3-mm-wide ridges (bars) spaced 1-20 mm apart. Surfaces were stroked across single fingertips at parametrically varied levels of force (25-150 g) and velocity (25-100 mm/sec). The average firing rates (AFRs) of many cells varied with Gw, but force and velocity altered response functions (e.g., from linear to plateau or inverted). Slowly adapting (SA) cells were more sensitive to force, rapidly adapting (RA) cells to velocity. Force and velocity affected all cells sensitive to Gw, which suggests that response independence (e.g., AFR correlated with Gw but not force or velocity) may require active touch. Discharge intervals of many cells replicated stimulus temporal period. This temporal fidelity in SAs far exceeded examples reported for active touch. However, discharge burst duration and AFR increased with Gw, supporting a neural rate rather than temporal code for roughness. Force and velocity altered the Gw at which some cells fired once in phase to stimulus cycle ("tuning point"). Responses to bar edges suggest cortical replication of peripheral mechanoreceptor sensitivity to skin curvature, leading to this temporal fidelity in some cortical cells. Graded RA responses to Gw without obvious stimulus temporal replication may reflect early stages of integrative processing in supra- and infragranular layers that blur obvious temporal patterning and lead to a rate code correlated with spatial variation and proportional to perceived roughness.

School adjustment in sixth graders: parenting transitions, family climate, and peer norm effects.

This study examined whether sixth graders' (mean age = 11.86 years) adjustment to the school context (assessed by grades, achievement scores, and disruptive behavior) was affected by factors from both the family context (represented by students' reports of the number of parenting transitions experienced as well as current levels of supervision, acceptance, autonomy granting, and conflict in the family) and the peer context (represented by students' perceptions of peer norms supporting academic excellence). School adjustment was related to the number of parenting transitions experienced, family climate, and peer norms as well as to higher-order relations involving family climate and peer norms such that (a) high grade point averages occurred at only moderate levels of family supervision, (b) achievement scores were positively related to supervision at only low levels of family autonomy granting, and (c) grade point average was positively related to peer norms at only high levels of family acceptance.

Representation of tactile roughness in thalamus and somatosensory cortex.

Neuronal responses were recorded in the thalamic ventroposterior lateral nucleus and primary and secondary somatosensory cortical areas of two rhesus monkeys performing a tactile discrimination task. The subjects actively stroked their fingertips over gratings that varied in groove width. Many cells in each location displayed average firing rates that incrementally reflected groove with dimensions (0.5-2.9 mm). Approximately 10% of cortical cells were more active to surfaces with narrower grooves. i.e., had negative graded response functions. All thalamic cells and approximately 50% of cortical cells with positive graded functions to gratings also showed increased responsiveness to contact force. Some cells also varied their activity with stroke speed. Many thalamic, a few primary somatosensory cortical cells, and no secondary somatosensory cortical cells showed periodic firing patterns that reflected the spatial-temporal frequency of stimulation. Responses to gratings of nearly every cell with negative graded responses and the remaining cortical cells with positive functions were independent of contact force and stroke velocity. The results only partially confirm predictions based on different models of texture perception that propose spatial, intensive, or cross modal neural codes. Negative graded response functions may require a form of spatial convergence across a cell's receptive field that has not previously been discussed by these models.

Neuronal activity in the second somatosensory cortex of monkeys (Macaca mulatta) during active touch of gratings.

1. In penetrations made into the upper bank of the lateral sulcus in two monkeys (Macaca mulatta), cells were isolated from the second somatosensory cortex (SII). During single-cell recordings, animals performed an active touch task in which they rubbed their fingertips over pairs of gratings differing in groove width and indicated which was the smoother surface. Hand motion and downward applied force were measured and recorded during these strokes. 2. In this survey, 151 penetrations provided observations on 352 cells that responded to passive stimulation of the digits or during performance of the active touch task. Consistent with previous reports, receptive fields (RFs) in SII were large, often multi-digit, and frequently included a portion or all of the hand and occasionally the arm. Modality was determined for 92 of 127 fully characterized cells, and included 70 cutaneous, 5 deep, 11 Pacinian corpuscle, and 6 joint cells. Characteristic of SII, modality could not be defined in 35 cells that were unresponsive to passive stimulation or whose responses varied widely over time. 3. Response properties of a subgroup of 79 cells in SII resembled those previously studied in the primary somatosensory cortex (SI) and ventroposterior lateral nucleus of the thalamus (VPL) using identical procedures. Correlation analysis revealed that 29 of these cells, like a portion of cells in SI, responded to changes in groove width independent of force or velocity. This selectivity could be considered a form of feature specificity. 4. In contrast to SI and VPL, transient responses to the fingertips contacting small elevated metal bars, which demarcated the beginning, middle, and end of strokes across the gratings, were seen in a majority of SII cells (109/127). During contact with bars, 89 cells displayed excitatory responses and 20 cells showed suppressed activity. Twelve cells, which responded to bars in isolation from gratings, provided a possible example of increased stimulus selectivity. 5. Passive stimulation failed to activate 16 cells that responded, in some cases differentially to gratings or force, during the task. Responses of nine other cells demonstrated task-dependent modulation in the form of response reduction or enhancement during selected portions of the stroke. In these same cells, response changes did not occur under comparable stimulus conditions in other portions of the stroke that differed only in behavioral context. These types of selective response modulations, not noted in our previous studies of VPL or SI, suggest that mechanisms regulating sensory inputs may affect SII.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuronal activity in the primary somatosensory cortex in monkeys (Macaca mulatta) during active touch of textured surface gratings: responses to groove width, applied force, and velocity of motion.

1. In a descriptive survey of primary somatosensory cortex (SI) responses during active touch, two monkeys (Macaca mulatta) were trained to stroke their fingertips over pairs of gratings with constant ridge (250 microns) and varying groove (500-2,900 microns) width (roughness) and to identify the smoother (smaller groove). Speed of hand motion and applied force level during the stroke were monitored and recorded. Transdural single-unit recordings were obtained from areas 3b and 1 of SI while animals performed the task. 2. The statistical sample consisted of 164 single units. Most cells in the sample responded in some fashion during the stroke, including brief increased or decreased activity to 1-mm ridges (touch strips) placed between surface pairs and at ends of each block to serve as touch sensors. These peristroke response patterns were described briefly. Most cells (153/164) responded to grating contact. Three types of responses to groove width were characterized. 1) Response was proportional to groove width in many cells. There was a vigorous response to roughest and none to smoothest surface. Mean firing rates for these cells appeared linearly related to groove width. 2) Graded responses were seen with smaller response differences to the same groove width range as in 1. Responses of some cells of types 1 and 2 were uncorrelated with variations in applied force and velocity of stroke. 3) There were inverse responses to groove width. Greater responses occurred during contact with smoother surfaces. 3. Many cells were influenced by a combination of changes in groove width, force, and/or velocity. Activity of a small sample of cells in area 1 with slowly adapting (SA) response properties was an almost exclusive positive function of variations in force level. Unlike SAs in 3b, responses of these cells were uncorrelated with alterations in groove width or stroke velocity. Velocity effects were almost always associated with response to groove width. Positive velocity cells coded temporal period. Significant velocity effects were not evident in graded or inverse graded cells. Negative force and velocity effects were due to shifts in behavioral strategy. Periodicity related to the spatial period of the grating was found in autocorrelograms of a small number of cells. Finally, responses of some cells were unaffected by changes in groove width, force, or velocity. Some of these were affected by contact with touch strips. Others responded in undifferentiated fashion to the stroke, and their function remains unresolved. Overall, there was a continuum of response patterns. Subgroups of cells were not distinct.(ABSTRACT TRUNCATED AT 400 WORDS)

Second somatosensory cortical area in macaque monkeys: 2. Neuronal responses to punctate vibrotactile stimulation of glabrous skin on the hand.

Single neuron responses from the second somatosensory cortical area (SII) of macaque monkeys were studied using computer-controlled vibratory stimuli ranging in frequency from 10 to 300 Hz. Results were obtained using chronic recording techniques in awake or lightly tranquilized animals. Most neurons were unable to follow the temporal order of vibrations in excess of 10 Hz. A smaller sample of cells provided faithful reproduction of frequencies up to 50-75 Hz and another responded to low amplitude, high frequency stimulation in excess of 100 Hz. Cells that displayed temporally cohesive responses to lower frequencies demonstrated predictable, time-locked discharges to successive stimulus cycles. Cells activated by higher frequencies showed a lower probability of following successive stimulus cycles. These findings are discussed in reference to the hypothesis that SII may provide a parallel channel for processing high frequency vibrotactile inputs from Pacinian receptors.

Tactile discrimination of gratings: psychophysical and neural correlates in human and monkey.

Human and monkey performance on discriminating tactile gratings revealed comparable cross-species Weber functions. Neural data obtained while monkeys performed discriminations revealed some matching of neural and psychometric functions. Nearly constant firing rate differences occurred at discrimination threshold for unequal groove widths. Firing rate differences of some cells decreased on trials preceding discrimination errors, and thus predicted performance.

Neuronal responses in ventroposterolateral nucleus of thalamus in monkeys (Macaca mulatta) during active touch of gratings.

Responses of single neurons were recorded from the ventroposterolateral nucleus (VPL) of the thalamus while a monkey stroked its fingertips over gratings. Monkeys were trained to stroke the gratings with consistent downward applied force and velocity of hand motion. Neurons were selected with receptive fields on the glabrous digits. Average firing rate was computed for a range of grating groove widths; groove width corresponded to roughness. Force and velocity were measured. VPL responses were compared to previously reported responses in primary somatosensory cortex (SI) under identical stimulus conditions, and to reports of peripheral afferent fiber responses to passively applied gratings. VPL responses more closely resembled those of peripheral afferent fibers than those of SI in important respects: lack of independent responses to roughness, force, and velocity; high temporal and force fidelity; and response patterns that closely followed the shape of elevated metal strips used to separate pairs of gratings. The presence in cortex of response patterns not seen in the thalamus, such as response independence and negative correlations to groove width, suggests that they stem from cortical processing.

Carbon Dioxide laser in the treatment of cutaneous disorders.

Experienced gained with the Carbon Dioxide Laser over a two year period is documented. This laser modality has been used successfully for many indications. We believe it is the treatment of choice for conditions such as resistant verrucae, extensive actinic cheilitis or labial leukoplakia, and for the cosmetic removal of widespread facial tumours, some keloid scars and rhinophyma.