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D M MacKay

Publications and source records attributed to D M MacKay.

At least 19 recordsLinked to original sources
PubMed1986

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.

Attention↗
PubMed1986

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.

Cerebral Cortex↗
PubMed1985

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.

Action Potentials↗
PubMed1983

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.

Electrodes↗
PubMed1983

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.

Animals↗
PubMed1979

Polarity-sensitive perceptual adaptation to temporal sawtooth modulation of luminance.

Exposure to a large uniform field modulated in luminance by a sawtooth function, repeating between 1 and 5 times per second, raised the threshold for detection of a test stimulus of similar waveform by a factor of 2 to 4.5. In comparison, the threshold elevation for a test stimulus of the inverse waveform was only half as great. This polarity-sensitive adaptation fits with Jung's hypothesis that separate channels signal 'brightening' and 'darkening' in the human visual system. Introduction of spatial contrast such as random noise does not affect adaptation to temporal luminance gradients, but does lead to some interocular transfer. The transferred component, however, shows no sensitivity to the polarity of the test stimulus.

Adaptation, Ocular↗
PubMed1977

Adaptation of evoked potentials by patterns of texture-contrast.

Visual evoked potentials (VEPs) to a briefly presented pattern in black-ahd-white are reduced by pre-exposure to the same pattern formed only by contrasting textures. When such texture-contrast patterns are used as stimuli, however, the VEP is different in latency and in its sensitivity to changes in pattern geometry.

Adaptation, Physiological↗