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D A Jeffreys

Publications and source records attributed to D A Jeffreys.

At least 19 recordsLinked to original sources
PubMed1992

The vertex-positive scalp potential evoked by faces and by objects.

The influence of stimulus form on the scalp-recorded "vertex positive peak" (VPP) evoked by images of faces (Jeffreys 1989a) was studied in seven subjects. In separate experiments, we recorded the responses to 2D images of: (1) many different depictions of human faces; (2) the heads of several different species; (3) many familiar non-face objects; and (4) stimuli where the configuration of objects were modified to produce an "illusory" or "non-contextual" subjective impression of a face. The results showed that every facial representation, including the "illusory" stimuli, and most of the non-face objects, evoked a VPP of corresponding form and scalp distribution. The object-evoked VPPs, however, were always smaller and usually later than those evoked by the faces. VPPs of longer latency but often comparable amplitude were also recorded for impoverished compared to well-defined facial representations; and for most non-human compared to human faces. Very consistent responses were recorded to repeated presentations of the same stimulus for the same subject, but there was considerable variation in latency as well as amplitude (but not form) of the VPP evoked under identical experimental conditions for different subjects. These response properties of the VPP, suggest that its underlying physiological generators are sensitive to basic configural properties of the visual stimulus; and also the face- and object-related information are processed in the same brain area(s), although not necessarily by the same physiological mechanisms.

Adult↗
PubMed1992

Evoked potential evidence for human brain mechanisms that respond to single, fixated faces.

The influence of visual fixation position and stimulus size on the scalp-recorded "vertex positive peak" (VPP) evoked by images of faces was studied in three subjects. Responses were recorded, in turn, for line-drawn, frontal-view faces of approximately 8, 4, 2, and 1 deg length, fixated at the centre (bridge of the nose), and at points 1, 2, 3, and 4 deg to the left and right, and above and below, centre. The results showed that central fixation produced VPPs of similar, maximal amplitude for all face sizes. By comparison, "on-face" eccentric viewing yielded attenuated and delayed responses, and the degree of response attenuation as a function of eccentricity was directly related to the face size, with similar amplitude responses being evoked for corresponding fixation locations on each face. Very small or no VPPs were recorded for most "off-face" fixations. Similar results were observed for profile faces, except that the maximal VPP was recorded for fixations near the eyes and not in the centre of the head, and almost identical VPPs were evoked by a centrally fixated face presented with and without an adjacent face or object. These response properties, which correspond to the subjective perception of the facial stimuli, suggest that the VPP reflects brain mechanisms optimized to respond to single, fixated faces, irrespective both of facial image size and of the presence of neighbouring figures.

Adult↗
PubMed1989

A face-responsive potential recorded from the human scalp.

Evoked potentials were recorded to the separate tachistoscopic presentation of a variety of faces and other simple and complex visual stimuli. A positive potential of 150-200 ms peak latency which responds preferentially, but not exclusively, to faces was identified in 8 out of 9 subjects. This potential, best recorded from midline central and parietal electrodes, was evoked by all face stimuli, including photographs, outline drawings, and fragmentary figures. Changes in stimulus size and other parameters which do not affect the clarity of the face, generally had little effect on the peak amplitude. Stimulus changes such as face inversion, reversing the contrast polarity of photographic images, and selectively removing particular facial features, produced a marked increase in latency but often only slight attenuation of this peak. These response properties correspond well with those reported for face-related single cells in the temporal cortex of the rhesus monkey. The scalp distribution of this face-responsive peak also appears consistent with bilateral sources in the temporal cortex.

Evoked Potentials, Visual↗
PubMed1986

A visual evoked potential study of metacontrast masking.

In a comparison of the subjective appearance of, and the scalp-recorded potentials evoked by, the first of two successively presented patterns of spatially adjacent elements, we recorded typical U-shaped metacontrast masking functions but found no discernible modification of either of the two initial VEP components, C1 and C2. These results, together with the previously established properties of C1 and C2, suggest that the early stages of contour-specific processing in the visual cortex have relatively short response latencies and durations and are uninfluenced by the subsequent presentation of metacontrast masking stimuli. The VEP data would thus appear to conflict with the basic assumptions of inhibition theories of metacontrast.

Evoked Potentials, Visual↗
PubMed1985

The influence of spatial frequency on the reaction times and evoked potentials recorded to grating pattern stimuli.

The simple reaction times recorded to sine-wave and square-wave grating stimulus patterns of both constant physical contrast and of constant suprathreshold contrast were appreciably delayed by an increase in spatial frequency from 0.5 to 10 c/deg. There was no comparable increase, however, in the peak latency of the initial visual evoked potential component, C1, recorded to the same stimulus patterns. In view of the evidence that C1 has a striate cortical origin, these results suggest that the large spatial-frequency dependent variations in RT do not reflect delays of stimulus-induced neuronal responses in the primary visual pathway from retina to striate cortex.

Evoked Potentials, Visual↗
PubMed1983

Visual evoked potentials to double-pulse pattern presentation.

The temporal resolution and summation characteristics of human cortical processes were investigated by recording an individual VEP component, C1, to the double-pulse presentation of pattern pairs of both the same (+ve/+ve; -ve/-ve) and of opposite (+ve/-ve; -ve/+ve) contrast polarity at varying onset-to-onset intervals (SOA). The results show that comparable limiting SOA values of 40-50 msec are needed for the C1 components to the two patterns to separate out to form a double-peaked response and for the stimulus to be seen as two distinct events. Also, the amplitude variations of the single-peaked response obtained at shorter SOAs show evidence of complete precortical response integration for SOA values below 5-10 msec and partial integration for values up to 30-40 msec, which again correspond to the results of related psychophysical studies. These VEPs show no reflection, however, of the inhibition/summation effects reported for subjective responses to pattern pairs of the same/opposite contrast polarity at SOA values between 40 and 70 msec. The implications of these findings are discussed.

Evoked Potentials, Visual↗
PubMed1982

Pattern-evoked potentials and Bloch's law.

Experiments are described which show that the latencies, unlike the amplitudes, of the C1 and C2 components of the human pattern-onset VEP do not conform to the contrast equivalent of Bloch's law. Whereas each component's amplitude is linearly related to the log of the (contrast X duration) product below a critical duration of 50 msec, its latency is independent of the stimulus pattern's duration and determined by its contrast alone; increasing by 30-35 msec for a 1.4 log unit contrast decrease. The relationship between these results and corresponding single-unit findings in the cat is discussed.

Evoked Potentials, Visual↗
PubMed1979

Evoked potential evidence for differences in binocularity between striate and prestriate regions of human visual cortex.

For two components of the visual evoked potential elicited by the onset of a stationary pattern the degree of interocular transfer of the attenuation caused by prior exposure to a similar pattern was measured. The results show almost complete interocular transfer for component CII, thought to originate in prestriate cortex, but only partial transfer of CI, thought to be of striate cortical origin. This suggests that in man, as in monkey, monocularly driven neurones are more common in striate than in prestriate cortex.

Dominance, Cerebral↗
PubMed1979

The polarity inversion of scalp potentials evoked by upper and lower half-field stimulus patterns: latency or surface distribution differences?

Evoked potentials to patterned stimulation of the upper and lower half of the visual field are generally inverted in polarity. Two conflicting proposals have been made to explain this effect, both based on surface distribution studies of pattern-reversal and/or pattern-onset VEPs. The first suggests that this polarity inversion is due to differences in surface distribution of corresponding components of constant latency; the second that it is due to differences in the latencies of peaks of similar surface distributions in the upper and lower half-field responses. Experimental evidence is here presented which supports the first explanation for the case of the pattern-onset VEPs. These results, which illustrate how different components in the same response can be identified from the selective adaptation effects of pre-exposure to outline patterns, show that there is no difference in latency of components of corresponding properties in the upper and lower half-field VEPs.

Electroencephalography↗