Using action understanding to understand the left inferior parietal cortex in the human brain.

Humans have a sophisticated knowledge of the actions that can be performed with objects. In an fMRI study we tried to establish whether this depends on areas that are homologous with the inferior parietal cortex (area PFG) in macaque monkeys. Cells have been described in area PFG that discharge differentially depending upon whether the observer sees an object being brought to the mouth or put in a container. In our study the observers saw videos in which the use of different objects was demonstrated in pantomime; and after viewing the videos, the subject had to pick the object that was appropriate to the pantomime. We found a cluster of activated voxels in parietal areas PFop and PFt and this cluster was greater in the left hemisphere than in the right. We suggest a mechanism that could account for this asymmetry, relate our results to handedness and suggest that they shed light on the human syndrome of apraxia. Finally, we suggest that during the evolution of the hominids, this same pantomime mechanism could have been used to 'name' or request objects.

Remembering John Newsom-Davis' contribution to human imaging in Oxford.

John Newsom-Davis played a crucial role in supporting areas of scientific exploration beyond his own research interests. In particular, he was one of the key players in establishing human neuroimaging in Oxford. Here, we celebrate the role that he played in this endeavour, both in the early days of pulling together funding, and solving practical challenges, and in the following years, when we all appreciated his ongoing encouragement and support.

Segregation of visual selection and saccades in human frontal eye fields.

The premotor theory of attention suggests that target processing and generation of a saccade to the target are interdependent. Temporally precise transcranial magnetic stimulation (TMS) was delivered over the human frontal eye fields, the area most frequently associated with the premotor theory in association with eye movements, while subjects performed a visually instructed pro-/antisaccade task. Visual analysis and saccade preparation were clearly separated in time, as indicated by 2 distinct time points of TMS delivery that resulted in elevated saccade latencies. These results show that visual analysis and saccade preparation, although frequently enacted together, are dissociable processes.

Delayed stoma formation in Fournier's gangrene.

Fournier's gangrene is traditionally treated with prompt surgical debridement and in many cases a diverting colostomy is also fashioned during the same procedure. We present four cases where stoma formation was delayed until the second look procedure. The physiological states at the time of either procedure were compared using POSSUM. The results showed an improvement in the physiological condition in all patients at the time of the second operation, suggesting that a delay can potentially improve prognosis in such cases.

Advances in broad bandwidth light sources for ultrahigh resolution optical coherence tomography.

Novel ultra-broad bandwidth light sources enabling unprecedented sub-2 microm axial resolution over the 400 nm-1700 nm wavelength range have been developed and evaluated with respect to their feasibility for clinical ultrahigh resolution optical coherence tomography (UHR OCT) applications. The state-of-the-art light sources described here include a compact Kerr lens mode locked Ti:sapphire laser (lambdaC = 785 nm, delta lambda = 260 nm, P(out) = 50 mW) and different nonlinear fibre-based light sources with spectral bandwidths (at full width at half maximum) up to 350 nm at lambdaC = 1130 nm and 470 nm at lambdaC = 1375 nm. In vitro UHR OCT imaging is demonstrated at multiple wavelengths in human cancer cells, animal ganglion cells as well as in neuropathologic and ophthalmic biopsies in order to compare and optimize UHR OCT image contrast, resolution and penetration depth.

Chromatic edges, surfaces and constancies in cerebral achromatopsia.

We tested achromatopsic observer, MS, on a number of tasks to establish the extent to which he can process chromatic contour. Stimuli, specified in terms of cone-contrast, were presented in a three-choice oddity paradigm. First we show that MS is able to discriminate the magnitude of chromatic and luminance contrast, but performance is inferior to that of normal observers. Moreover, MS can discriminate isoluminant borders of different chromatic composition. These abilities are not the result of unintended luminance differences and are abolished when chromatic borders are masked by sharp luminance change. In simple displays, local cone-contrast signals can make a significant contribution to surface colour appearance in normal observers. In more complex displays, the perception of a surface's colour becomes largely independent of the local contrast to its background, via processes presumed to be similar to the edge integration and anchoring stages of Land's Retinex algorithm. We show that in simple displays the percepts of both MS and normal observers are dominated by local chromatic-contrast. But, although the percepts of normal observers change in line with the predictions of retinex theory in more complex displays, those of MS do not, remaining dominated by local contrast signals. We conclude that MS has lost the ability to perform edge integration and that this loss is closely related to his absence of colour experience.

A review of 187 gunshot wound admissions to a teaching hospital over a 54-month period: training and service implications.

BACKGROUND: Violence involving the use of firearms has increased in the UK over the past decade. This study assesses the implications of such injuries for service provision and training by reviewing the experience at one hospital. METHODS: Accident and emergency triage data were searched for patients presenting with gunshot wounds over a 54-month period. Case notes were reviewed and patterns of care established. The resources required for clinical management were ascertained, and the financial consequences determined at contemporary full cost. RESULTS: There were 187 attendances with 247 wounds. Mean age was 21 years (range, 8-63 years). Of the attendances, 69% were out of normal working hours. Of the 187 cases, 97 patients were admitted to one hospital (83 of whom required surgery) and 10 patients were transferred to other hospitals (6 for plastic surgery not available at the Manchester Royal Infirmary and 4 due to lack of beds). Of the 80 patients who were not admitted, 4 died in accident and emergency, the rest were either air gun wounds or relatively simple higher calibre injuries. A wide range of surgical specialties was involved (limb injury, 53; thoraco-abdominal and vascular, 28; head and neck, 5; and orbit, 2), and combinations of injuries transgressed specialty and sub-specialty boundaries. The total cost of patient care was pound 267,000. CONCLUSIONS: Gunshot wounds present a heavy demand on the clinical and financial resources of the receiving hospital, and surgeons responsible for unselected acute admissions in "general surgery" should be capable of dealing with these indiscriminate injuries. Current training and service trends towards increasing sub-specialisation may mitigate against them achieving or retaining this capability.

Brain potentials associated with conscious aftereffects induced by unseen stimuli in a blindsight subject.

The study is of brain activity in a blindsight subject (D.B.), who reports conscious visual afterimages of stimuli of which he is unaware when they are presented. This contrast offered a unique opportunity to study event-related potential recordings of conscious versus unconscious visual phenomena generated by the very same stimulus in the identical locus of the visual field. The behavioral results confirmed the reliability of the difference in the subject's report for inducing stimuli versus their aftereffects. The rationale of the event-related potential analysis was to subtract "on" signals from "off" signals, the latter associated with the onset of conscious events and the former for events that remained unconscious. Because there are inherent differences in on and off potentials, the subtractive resultants for the blind hemifield were compared with the same subtractions for the good hemifield when the subject was aware both of the stimuli and their afterimages. A differential pattern in subtractive resultants emerged with a strong anterior left frontal focus for the blind field and a posterior focus for the intact field. The results are compared with other studies suggesting an anterior focus for conscious visual events.

Compact, low-cost Ti:Al2O3 laser for in vivo ultrahigh-resolution optical coherence tomography.

A compact, low-cost, prismless Ti:Al2O3 laser with 176-nm bandwidth (FWHM) and 20-mW output power was developed. Ultrahigh-resolution ophthalmic optical coherence tomography (OCT) ex vivo imaging in an animal model with approximately 1.2-microm axial resolution and in vivo imaging in patients with macular pathologies with approximately 3-microm axial resolution were demonstrated. Owing to the pump laser, this light source significantly reduces the cost of broadband OCT systems. Furthermore, the source has great potential for clinical application of spectroscopic and ultrahigh-resolution OCT because of its small footprint (500 mm x 180 mm including the pump laser), user friendliness, stability, and reproducibility.

Visual deficits in a patient with 'kaleidoscopic disintegration of the visual world'.

We describe psychophysical, neuropsychological and neuro-ophthalmological studies of visual abilities in a patient who, following a right hemisphere stroke, had difficulty in combining parts of objects into a whole and in reading. Strikingly, her perceptual problems were accentuated when the objects moved or when she moved. Formal testing showed that her main deficits were in depth perception, various tasks of motion and object recognition of degraded stimuli. But low-level detection and discrimination of form and color were normal. Despite her deficits in visual motion and degraded static-object recognition, her visual recognition of 'biological motion' stimuli was normal. Structural magnetic resonance imaging revealed an infarct in the ventro-medial occipito-temporal region, extending ventro-laterally and leading to a 'kaleidoscopic disintegration of visible objects'.

Tickling the brain: studying visual sensation, perception and cognition by transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) is a means of stimulating the brain from outside the skull with little, and occasionally no discomfort for the subject. A single TMS pulse, lasting less than 1 ms, can briefly disrupt the normal activity of a targeted region of the brain for tens of milliseconds, allowing the effects of disruption on specific perceptual and cognitive tasks to be measured behaviorally. Rapid, repeated pulses can disrupt activity for correspondingly longer periods. The reversibility of the effects make it possible to create 'virtual patients' who can be tested in the same way as actual patients with real brain damage in order to explore regional functional specialization. Although several aspects of TMS continue to be evaluated, such as its safety, the extent and localization of the effective region of induced electrical current, the importance of the waveform of the pulse, the configuration and positioning of the coil, its productivity has been firmly established in little more than 10 years of systematic use. Examples of the latter are given from investigations of the nature of visual phosphenes produced by TMS applied to different regions of the visual cortex in normal subjects and subjects with occipital or ocular damage in an attempt to reveal the role of visual cortex in visual awareness.

Detection and discrimination of chromatic targets in hemianopic macaque monkeys and humans.

We measured the ability of three macaque monkeys with unilateral removal of primary visual cortex to detect 9 degrees, 200-ms targets presented at random in the upper or lower quadrants of the normal and hemianopic visual fields. The white or variously coloured target could differ from the background in both colour and luminance, or in either of them. Blue and red targets were detectable at any luminance contrast, but green and white targets were barely or not at all detectable at and near isoluminance in the hemianopic field. Blue and red targets were better detected than white targets at the same luminance difference. However, when both the target and the background were dynamically luminance-masked, detection in the hemianopic field failed at isoluminance whatever the colour. In addition, the monkeys were unable to discriminate between simultaneously presented red and green or blue and green targets in the hemianopic field when both targets had similar luminance contrast with the background. Two hemianopic patients tested on a subset of the tasks performed similarly to the monkeys. Together, the results indicate that the residual colour-opponent system that survives damage to V1 is involved in the detection of chromatic changes but cannot sustain simultaneous discrimination between pairs of colours.

The regional cortical basis of achromatopsia: a study on macaque monkeys and an achromatopsic patient.

Previous experiments have revealed total loss of colour vision following removal of all inferior temporal cortex, a condition akin to complete cerebral achromatopsia in humans. Whether less extensive ablation genuinely impairs colour perception without abolishing it or retards learning involving coloured stimuli is contested. We therefore tested macaque monkeys, with total removal of temporal areas TEO and TE but sparing rostral and perirhinal temporal cortex and the upper bank of the superior temporal sulcus. Compared with three monkeys with lateral parietal ablations, the monkeys with TEO/TE lesions were impaired at learning and retention of simultaneous two-choice colour discriminations and with a nine-choice oddity discrimination whether the coloured target was embedded among grey distracters of the same luminance or among isoluminant coloured distracters. However, their performance was superior to that of an achromatopsic human subject and to that previously measured in monkeys with much larger temporal lobe ablation. They were only mildly impaired at nine-choice oddity discrimination for grey stimuli where the grey target was brighter than the grey distracters. The impairment could be exacerbated or alleviated by altering the colour of the background of the displays and by static and dynamic luminance masking of the entire display in a manner that indicates that the colour deficit reflects a change in perception rather than a disorder of learning and memory. It resembles central dyschromatopsia in human subjects but falls short of achromatopsia.

Transcranial magnetic stimulation. Which part of the current waveform causes the stimulation?

To investigate the mechanism of transcranial magnetic stimulation (TMS), we compared the directional effects of two stimulators (Magstim 200 and Magstim Super Rapid). First, stimulating visual cortex and facial nerve with occipital mid-line TMS, we found that, for a particular coil orientation, these two stimulators affected a particular neural structure in opposite hemispheres and that, to affect a particular neural structure in a particular hemisphere, these two stimulators required opposite coil orientations. Second, stimulating a membrane-simulating circuit, we found that, for a particular coil orientation, these two stimulators resulted in a peak induced current of the same polarity but in a peak induced charge accumulation of opposite polarity. We suggest that the critical parameter in TMS is the amplitude of the induced charge accumulation rather than the amplitude of the induced current. Accordingly, TMS would be elicited just before the end of the first (Magstim 200) and second (Magstim Super Rapid) phase of the induced current rather than just after the start of the first phase of the induced current.

The role of transcranial magnetic stimulation (TMS) in studies of vision, attention and cognition.

Transcranial magnetic stimulation (TMS) can be conceptualized as a virtual lesion technique, capable of disrupting organized cortical activity, transiently and reversibly. The technique combines good spatial and temporal resolution and, moreover, because it represents an interference technique, can be said to have excellent functional resolution. The following is a review and discussion of the contribution which TMS has made to the study of vision, attention, development and plasticity and speech and language.

Pattern electroretinograms after cerebral hemispherectomy.

Cortically blind patients with brain damage restricted to the optic radiations or primary visual cortex may be able to detect and discriminate visual stimuli presented in their field defects, even though they deny seeing them. In contrast, patients who are hemianopic as a result of cerebral hemispherectomy cannot explicitly discriminate visual stimuli in their field defects, even when forced choice procedures are used. A possible explanation for this difference is that retrograde, transneuronal degeneration of the retina, which affects approximately 85% of wavelength-sensitive ganglion cells (approximately 70% of the total) after damage restricted to striate cortex, could be far more extensive after hemispherectomy, rendering the retina incapable of processing and conveying visual information to the brain. To test this, we assessed retinal ganglion cell function by means of electroretinography in three patients with cerebral hemispherectomy who were functionally blind. Steady-state pattern electroretinograms elicited by achromatic and isoluminant-chromatic (red-green) sinusoidal gratings, whose contrast was temporally modulated, were recorded from both blind and sighted hemiretinae. The electroretinograms were qualitatively indistinguishable from those of a control patient with a unilateral striate cortical lesion with documented visual capacity in his field defect. Within-subject analysis of variance revealed significant differences in the amplitude of the second harmonic (2f(0)) component of the averaged signal (diagnostic of retinal ganglion cell function) with respect to stimulus, but no significant differences between blind and sighted hemiretinae. This indicates that many retinal ganglion cells must have survived in the hemispherectomized patients. Isoluminant chromatic stimuli tended to elicit stronger signals than achromatic stimuli, which was unexpected given that wavelength-opponent Pbeta ganglion cells are far more susceptible than broad-band Palpha ganglion cells to transneuronal degeneration after cortical damage. It suggests that the 2f(0) component of the response to isoluminant chromatic stimuli might not reflect the activity of chromatic processes. Overall, the results show that the absence of residual vision in the blind fields of patients with cerebral hemispherectomy cannot be due to complete degeneration of retinal ganglion cells and, by extension, complete degeneration of their subcortical targets. This supports an alternative explanation, which is that intact extrastriate cortex is required for mediating voluntary responses to visual stimuli presented in the scotoma.

Regional cerebral correlates of global motion perception: evidence from unilateral cerebral brain damage.

We used a psychophysical task to measure sensitivity to motion direction in 50 stroke patients with unilateral brain lesions and 85 control subjects. Subjects were asked to discriminate the overall direction of motion in dynamic stochastic random dot displays in which only a variable proportion of the spots moved in a single direction while the remainder moved randomly. Behavioural and neurophysiological evidence shows that the middle temporal (MT/V5) and middle superior temporal (MST) areas in the macaque monkey are indispensably involved in the perception of this type of motion. In human subjects too, lesions in the same region disrupt performance on this task. Here we assessed more extensively the correlation between direction sensitivity for global motion and the anatomical locus of the lesion. Thresholds for perceiving the direction of global motion were impaired in the visual field contralateral to the lesion in patients with lesions in the occipitoparietal and parietotemporal areas involving the human analogue of areas MT/V5 and MST, but not by lesions in the occipito-temporal or anterior frontal areas. Patients with lesions involving the anterior temporal or parietal lobes displayed poor performance for stimuli presented in either visual field, which is consistent with the large and bilateral receptive fields in these areas in monkeys. The perception of global motion was also more impaired in the centripetal than the centrifugal direction in the hemifield contralateral to the MT/V5 lesion. Surprisingly, thresholds were normal in all patients when the displays contained static but not dynamic visual noise, suggesting that their deficit reflects an inability to filter out dynamic noise. Although frequent repeated testing of some patients whose lesion involved the human homologue of MT was accompanied by an improvement in performance, this was no greater than in other patients who received training on different motion tasks.

The retinal projection to the pregeniculate nucleus in normal and destriate monkeys.

Taking advantage of the segregation of different classes of ganglion cell fibres in the optic tract, we investigated the ganglion cell class that projects to the pregeniculate nucleus (PGN) in the normal macaque monkey (Macaca mulatta and Macaca fascicularis) and following long-standing removal of striate cortex in one hemisphere. Confining small pellets of horseradish peroxidase and biocytin into ventral or dorsal parts of the tract unilaterally, or placing several pellets throughout the tract, we labelled the retina retrogradely and the PGN anterogradely. Classification of ganglion cells according to soma size and dendritic morphology showed that implants in the dorsal part of the tract labelled predominantly, and perhaps exclusively, P beta cells, and produced dense anterograde label in the parvocellular lateral geniculate nucleus as well in the PGN. Labelling of the PGN was sparse or absent following implants into the ventral aspect of the tract, which labelled the magnocellular geniculate nucleus anterogradely and chiefly P alpha and P gamma cells retrogradely. The finding of a projection to the PGN from P beta cells has implications for the permanent selective sparing of a subpopulation of these cells after removal of striate cortex and their contribution to wavelength processing in blindsight.

Motion discrimination in cortically blind patients.

Some patients with brain damage affecting the striate cortex, though clinically blind in their field defects, can still discriminate visual stimuli when forced choice procedures are used. Such patients seem particularly sensitive to moving stimuli in their scotomata, though there are conflicting reports as to whether they can discriminate the direction of motion. We tested three patients with areas of cortical blindness for their ability to detect and discriminate the direction of motion of a variety of first-order motion stimuli, namely bars, gratings, plaids and random dot kinematograms depicting translation and motion in depth, during forced choice tasks. The patients could detect the presence of movement in any kind of stimulus, and could discriminate the direction of single bars, but none could discriminate the direction of motion of the more complex stimuli (gratings, plaids and random dot kinematograms) or discriminate between 0 and 100% coherent random dot kinematograms at any speed tested (from 4 to 64 degrees /s). Similar results were obtained from one of the patients who was additionally tested with second-order versions of the translated bar and random dot kinematograms, eliminating light scatter as an explanation. Overall, the results suggest that motion processing in the scotoma is severely impaired, and that the puzzling discrepancies between previous studies can be accounted for by the type of stimulus used. The motion discrimination impairment caused by brain damage affecting the primary visual cortex is inconsistent with the proposed existence of a subcortical pathway to extrastriate cortical motion areas (such as areas MT and MST) which bypasses the striate cortex and is specialized for analysing 'fast' motion.