Scalp topography of ERP source-densities during visually guided target practice.

"Whole-head" maps of ERP source densities were recorded at 42 locations, spaced radially at 6 cm in hexagonal arrays, from two human subjects (S) during a visually-guided target-practice task with manual button-pressing and visual feedback, in a series of six or more 9 h recording sessions. From one S a finer-grained map (3 cm spacing) was also obtained at 13 occipital placements. Corresponding results at a smaller number of locations were obtained from nine other Ss. The topographical effects of changing the hand used, reversing the direction and hemifield of target approach, and changing the difficulty of the task were investigated in detail. Only locations near and just anterior to Cz were significantly affected by exchanging hands. Contralateral preparatory negativities, and ipsilateral negativities in anticipation of feedback, were enhanced. Changing the stimulus hemifield mainly affected occipito-parietal locations, with enhancement of contralateral preparatory-phase negativities, and of ipsilateral pre-feedback negativities, both there and near Cz. At certain parietal locations, however, the profiles were unaffected. Increase of task difficulty enhanced the difference between evaluation potentials for success and failure at a few locations, but left the general map largely unaltered. When comparing topographic features of the source-density maps in the two main Ss, the standard 10/20 topographic landmarks proved surprisingly unreliable. For each phase of the task, there were also hemispheric asymmetries not linked to the hand used or the hemifield stimulated; but their topography varied from S to S. Source densities during the preparatory phase were strongest around the vertex, with accompanying activity in parietal as well as occipital areas. No endogenous predictors of success and failure could be seen in the preparatory source-density profile. The source density map of the evaluation potential elicited by feedback suggests a relatively deep location for its generator. The ERP correlates of success and failure differ mainly in their form; amplitude differences vary from S to S. When S participates only imaginatively as a spectator of target practice by others or by machine, the general form of the ERP map is similar over the whole cycle, but amplitudes diminish from rear to front of the head. The cognitive significance of the main ERP features is confirmed by their virtual disappearance, except near the occipital poles, when S's attention is distracted from the significance of the visual input.

Source density analysis of scalp potentials during linguistic and non-linguistic processing of visual stimuli.

Event-related potentials (ERPs) were recorded from 40 locations, covering most of the scalp, during repeated tasks in which the observer (O) had to judge either the tense of a printed verb (V) or the symmetry of a spatial pattern (S). Stimuli were drawn at random from large ensembles. A simplified method of Laplacean analysis (MacKay 1983, 1984) allowed the corresponding source densities to be mapped at up to 28 locations, relatively free of artefacts due to eye movements or tongue movements. O signalled his judgement in each case by pressing one of two buttons on a given cue. The decision time allowed was kept short (about 1 s) but long enough for the task to be handled successfully. When stimuli 'V' and 'S' were drawn from geometrically different ensembles, the source-density distributions for the two tasks differed significantly at a number of locations. When 'V' and 'S' were drawn from a common ensemble, however, and O was instructed on each trial (in random order) to assess each stimulus as a word or as a geometrical pattern, the similarities in the source-density maps were more striking than the differences. It would seem that during sufficiently rapid verbal and spatial judgments, little sign of hemispheric specialization or task-specific differences may appear in the spatiotemporal profile of ERP source densities. More salient differences, some lateralized, appeared during the preparation interval prior to verbal and spatial tasks; but their pattern varied widely from subject to subject.

Influence of luminance gradient reversal on complex cells in feline striate cortex.

The effects of reversing the polarity of luminance contrast in adjacent segments of a bar stimulus have been investigated in complex cells of area 17 in cats lightly anaesthetized with nitrous oxide/oxygen halothane mixtures. On the basis of length summation behaviour, complex cells were classified as standard (length summating) or special (optimum response to a bar much shorter than the receptive field), after Gilbert (1977), and were further subdivided into groups lacking and possessing end-stopping. For each type of complex cell, we measured the effects of adding short segments of one polarity of contrast (light or dark) to either end of a bar of fixed length and optimum orientation but of opposite contrast. In all cells the response to the central bar was depressed by short segments of reversed contrast to an extent greater than predicted from the cells' length summation characteristics. Responses were minimized or abolished at a critical segment length. Increases beyond the critical length elicited a progressive recovery in response to a plateau level. In end-stopped cells this was followed by a further decline in response up to the limits of the cells' inhibitory end-zones. Special and standard complex cells differed only in their susceptibility to reversed-contrast segments above the critical length. In standard complex cells, the recovery of response matched the cells' length summation profiles in slope and cut-off point. In special complex cells the recovery was flatter in slope and significantly more protracted than the length summation profile. Similar results were obtained for either direction of motion (orthogonal to a cell's optimum orientation) and for either polarity of contrast (dark centre, light ends or the reverse). As might be expected, all the effects were weighted in favour of the receptive field centre. Thus the upturn in response as reversed-contrast segments were progressively extended was more rapid and achieved a higher limiting level when the central bar was short. Merely interrupting the contours of a bar by a central gap, rather than a segment of reversed contrast, gave rise to no more attenuation of response than that predictable from the length summation curve. The results are compared and contrasted with our comparable data for simple cells.

Source-density mapping of human visual receptive fields using scalp electrodes.

A triangular array of 20 electrodes spaced by 1.5 cm was used to record the distribution of current source-densities at 12 locations over the occipital scalp, in response to tachistoscopic presentation of a 1/2 degrees X 1/2 degrees scanning pattern element which explored a 2 degrees X 2 degrees area of the visual field. Single scalp locations had visual receptive fields of the order of 1 degrees-2 degrees in diameter, their shape varying somewhat according to the response in question. Source density analysis can resolve details of the AEP scalp map to better than 1 cm. Even with a 1/2 degrees X 1/2 degrees stimulus, averages of only a few tens of sweeps gave adequate signal-to-noise ratios.

Source density analysis of scalp potentials during evaluated action. I. Coronal distribution.

The distribution of source densities (the "Laplacean") of event related potentials (ERPs) over the scalp during a goal-directed task has been derived by a novel method which presents seven samples in a hexagonal array, using an economical computing technique that affords much freedom from artefactual contamination. The task had four phases: waiting for a ball to appear, observing its coordinates and estimating action needed to get it through a goal-mouth, acting and awaiting the outcome, and finally evaluating the outcome as success or failure. Different phases of the task were characterized by significantly different distributions of sources. Failures were distinguished from success to consistent features of the last phase of the ERP. All locations within the reference hexagon (spanning a circle of approximately 12 cm diameter on the scalp) showed the latter features. In control experiments, the effects of response mode and of different cognitive interpretations of the input/output situation were studied. A check showed that with monopolar recording involuntary tongue movements can totally vitiate conclusions in such investigations. The Laplacean derivations proved relatively free of such artefactual contamination.

Source density analysis of scalp potentials during evaluated action. II. Lateral distributions.

An economical method of computing the source densities underlying human scalp-recorded event-related potentials (ERPs) is used to investigate the topography of "evaluation potentials" elicited when a subject assess the outcome of a goal-directed activity. No sign of a generative focus was observed in frontal locations; but lateral placements over temporal and parietal lobes revealed an inversion of polarity of the evaluation potential suggesting the involvement of those regions in its computation. Experiments using auditory feedback of outcome, instead of or together with visual, showed no qualitative dependence of the main phenomena on the modality of feedback.

Do 'evaluation potentials' reflect cognitive assessment?

Event-related potentials were recorded when a subject evaluated the outcome of a simple 'TV game' as successful/unsuccessful, where the 'goal' was specified randomly as one of two areas on the screen. The 'evaluation potential' elicited by the outcome was consistently larger for unsuccessful outcomes, regardless of the location of the goal.

On-line source-density computation with a minimum of electrodes.

An economical arrangement is described whereby the Laplacean of the scalp potential distribution delta2V/delta X2 + delta 2V/delta y2 can be approximately computed by analog methods, using a simple adaptation of the input stages of conventional EEG amplifiers. For maximum efficiency the method requires electrodes to be disposed in equilateral triangular arrays. This makes possible the computation of approximate source densities at 7 scalp locations for a total of only 13 electrodes.

Influence of luminance gradient reversal on simple cells in feline striate cortex.

1. The sensitivity of simple cells to luminance gradient reversal in bar stimuli has been investigated in the striate cortex of lightly anaesthetized cats.2. The influence of segment(s) of one polarity of contrast, partially masking or added end-on to a bar of reversed contrast, was assessed against a stationary textured background (of intermediate average luminance), which was itself without influence on cell behaviour.3. In either configuration, short segments of reversed contrast were suppressive of bar response, to an extent varying with location along the receptive-field axis, but much greater than predictable from length-summation characteristics. Response suppression, even by very short segments, was often total.4. The effects of longer segments, added end-on to a bar of opposite contrast, depended on the extent of length summation exhibited by each cell. With progressive extension of these segments, some recovery of response occurred in a few simple cells with larger receptive fields, but not in small-field simple cells. The behaviour of end-stopped simple cells was comparable in all respects to that of their end-free counterparts, within the length-summation zone; thereafter, invasion of the inhibitory end-zones by bars of either polarity elicited a generalized decline in response.5. Where responsiveness was restored by adding longer segments of opposite polarity, the response peak shifted to the discharge centre appropriate to that polarity.6. All these results were found with either polarity of contrast, i.e. a light bar plus dark segments or the converse.7. Similar results were obtained for motion in either direction across the receptive field; and also for flash-presentation, over either the centre or flanks (whether ;on' or ;off') of the receptive field.8. The effectiveness of contrasting added segments was assessed as a function of location along the receptive-field axis for flash-presented and for moving stimuli. Suppressive effects were greatest over the centre of the receptive field, declining progressively in either direction along its axis, but over greater distances than anticipated from length summation.9. None of the results presented could have been predicted from length-summation characteristics. They are strongly suggestive of gating, rather than linear, antagonistic interactions.

Sleep and the McCollough effect.

Orientation-contingent chromatic ("McCollough") aftereffects (OCCAs) were induced under carefully standardised conditions, in subjects suffering from varying degrees of sleep deprivation. The initial OCCA strength was found to vary systematically with the prior sleep pattern of the subject. In subjects whose sleep normally lasted 7-8 hr, loss of 3 or 4 hr of the previous night's sleep could reduce initial strength by as much as 50%; no further reduction was observed with still shorter sleep durations. Keeping the eye in darkness while awake had in this respect no comparable effects to those of sleep, and sleeping in a lighted room made no difference. The results suggest that the McCollough Effect depends on a form of neural plasticity which requires a normal sleep cycle for its maintenance.

Effects of luminance gradient reversal on complex cells in cat striate cortex.

We have examined the responsiveness of various classes of complex cells in the cat's striate cortex to stimuli comprising bars aligned with segments of opposite luminance contrast. In all cases, a short segment of opposite polarity depressed, without abolishing, the bar response; within the length summation zone, the response to the bar in the presence of longer segments matched the length summation characteristics of each cell, depressed by a constant amount. The results could not be accounted for purely on the basis of convergent input from simple cells, whose behaviour to comparable stimuli has previously been reported (Hammond and MacKay 1981a).

Short-term reorganization of the cortical network? Some questions from visual psychophysics.

Research into visual system function requires the use of test stimuli as (would-be) neutral probes. Although the risk of transient 'fatigue' is well recognized, e.g. in relation to dark adaptation, we tend otherwise to assume that the system will respond reversibly to stimuli within normal physiological limits. It is on this assumption that gratings of near-parallel lines, for example, are commonly used to determine both physiological and psychophysical response characteristics. This paper reviews evidence suggesting that certain classes of visual stimuli, including gratings in particular, can induce a short-term cooperative reorganization of the visual network that leaves it far from normal in its responsiveness to other inputs. It is suggested that the resulting abnormalities may help to shape our ideas as to the cooperative ensemble properties of cortical neuronal network.