The temporal structure of visual stimulation and evoked response

D. Papakosiopoulos,

ed. / Psychophysiology

Society Abstracts,

293

1982

group H gave smaller NlOO responses to the nontarget stimulus than did group L. In addition, group H gave P300 responses to the target stimulus that were smaller, peaked later, and were followed by a more prolonged period of positivity than were those of group L. Group differences were not detected by measures either of accuracy of phoneme discrimination, or of video game performance (subject’s score on a given trial, in both raw and standardized forms). The NlOO results obtained during divided attention are consistent with recent research (Jutai and Hare, in press) suggesting that psychopaths effectively ignore stimuli that are not of immediate interest. The P300 findings suggest that the demands of a dual-task may have greater impact on post-decisional aspects of information processing in psychopaths than in other criminals.

Reference Jutai, J.W. and Hare, R.D. (in press). Psychopathy and selective attention complex perceptual-motor task. Psychophysiology.

THE TEMPORAL STRUCTURE EVOKED RESPONSE A. JOHNSTON Department of

during performance

OF VISUAL STIMULATION

of a

AND

and M.J. WRIGHT

Psychologv,

Brunei

Uniuerslty,

Uxbridge,

UK

Regan (1977) defines a transient visual evoked potential (VEP) as the response to a visual stimulus with a pulse or step temporal waveform, and a steady-state VEP as the stable response to a long train of regular, repetitive stimuli. Conventionally, responses to repetitive step changes, e.g. 2Hz patternreversal (PR), are treated as transient and analysed as voltage/time graphs, whereas responses to sinusoidal counterphase modulation (SC) or rapid PR are treated as steady-state and often analysed into frequency components. Responses were compared to 2Hz square-wave counterphase (PR) and sine-wave counterphase (SC) modulated gratings as a function of spatial frequency and monopolar occipital electrode location. Similar spatial frequency tuning curves and scalp distributions, and similar degrees of inter-subject variation were found for both conditions and both types of analysis. Fourier transforms of the average VEPs showed that the largest response to both PR and SC was to the

294

D. Papakostopoulos,

ed. / Psychophysiology

Socrety Abstracts.

1982

second harmonic of the modulation frequency, and all the energy was found in even harmonics. Thus for 2Hz PR the response is not ‘transient’ in the strict sense of Regan’s definition since frequency components are harmonically related to repetition frequency rather than to a step or impulse function. data indicate a rectifying nonlinearity in the VEP.

The

Reference Regan D. (1977). Fourier analysis of evoked potentials: In: Desmedt, J.E. (Ed.). Visual Evoked Potentials Press: Oxford, 110-120.

Some methods based on Fourier analysis. in Man: New Developments. Clarendon

AN ELECTROCORTICAL INVESTIGATION IN A BACKWARD MASKING PARADIGM Daniel

OF WORD RECOGNITION

U. BRANDEIS

Department

of Psychology,

University of British Columbia,

Three aspects of stimulus content,

Vancouoer, B.C., Canada

i.e. meaningfulness,

familiarity

and task

relevance, were manipulated without the subjects awareness. A number of subliminal (backward masked) stimuli were presented to the subject whose task it was to estimate an interval of 1 set (starting with the presentation-flash) by pressing a button. Supraliminal words were randomly interspersed among these subliminal stimuli, appearing above or below the masked field. Whenever the subject detected a previously assigned target among the supraliminal stimuli, she/he was required to press a button as fast as possible. The meaningfulness of the subliminal material was manipulated using words, non-words

and blanks.

Three

groups

of words

were used;

the targets,

the

nontargets and other ‘new’ words (which were never resented supraliminally). Task relevance, (target vs. nontargets) and familiarity (‘new’ words vs other words) were thus manipulated. Unexpectedly, detection performance was better with words than with non-words. This suggests that detection involves a late process drawing on lexical information. Several components of the event related potential (ERP) differentiated as early as 140 msec post-stimulus between sub- and supraliminal conditions. More importantly, differences within the subliminal conditions were observed; familiarity was discriminated after 260 msec and simple presence of a string after 300 msec. These results are