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Neurocognitive models of visual object identification have focussed on processes at the moment of identification, when perceivers can actually name what they see. Less well known is the timecourse of processes preceding and leading to actual identification. To track neuromental processes involved in visual identification, behavioral measures and event-related potentials (ERPs) were recorded in two experiments prior to, during and after the identification of fragmented objects, half of which had been shown in their complete versions in a previous study phase. Each object was revealed in a sequence of frames wherein the object was represented by an increasingly less and less fragmented image up to the complete version. A shift in ERPs, around 300 ms and beyond, from negativity to positivity, marked the transition from non-identification to identification. However, while for new stimuli such a shift appeared abruptly from non-identification to identification, for recently-studied objects a late positive wave emerged in response to unidentified fragments at a level just prior to overt identification. Thus, ERPs reflected covert processes associated with a successful match between the current visual information and episodic recently-stored memory traces, which predicted overt identification.

To investigate basic visual information processing in patients with hemineglect syndrome, pattern reversal visual evoked potentials (VEPs) were recorded in 21 brain-injured patients (10 with neglect symptoms) and 6 healthy subjects. The stimulus was a checkerboard which varied in check size or temporal frequency, presented to the left or right visual field. VEPs recorded in neglect patients to stimuli presented in the subjectively neglected left visual field were comparable in amplitude to those recorded to stimuli presented in the normal right visual field. For stimuli presented centrally, there was no difference in VEPs between neglect patients and brain-damaged patients without neglect. These results support neuropsychological theories that state that the neglect syndrome is more closely related to deficits at post-sensory levels than to impairments in basic visual processing. Some evidence, however, suggests that the nature of the interaction between the two visual hemi-fields may also be altered in neglect patients.

In order to enhance the effect of spatial frequency on the hemispheric asymmetry of visual evoked potentials (VEP), the response amplitudes to ON-OFF modulated gratings were compared with the responses to pattern reversal stimulation. Sinusoidal gratings of different spatial frequencies were presented to six righthanders. VEPs were recorded from temporal leads on each hemisphere. In the left hemisphere, the amplitude was constant for the two modes of presentation and independent of spatial frequency. In the right hemisphere, the response amplitude was larger to the ONSET stage of ON-OFF stimulation than to reversal and presented the characteristic spatial frequency tuning curve. This asymmetry is assumed to reflect a difference in sensitivity of the two hemispheres to the spatiotemporal characteristics of the stimulus. The relevance of these findings is discussed in relation to the other hemispheric specialization models.

Pattern-reversal visual evoked potentials (EPs) were recorded in seven adult subjects to assess the existence of rhythmic variations in processing visual information at a primary stage. During a session of two hours, sixty EPs were recorded on the left and right cerebral hemispheres. The amplitude of N75-P100 component was measured. Remarkable variations were found with a periodicity from 15 to 60 min in six out of seven subjects. No hemispheric differences were found in the rhythmic variations. The results are in agreement with the behavioral data which show ultradian variations in visual performance depending on a general activation oscillator.

Visual evoked potentials were recorded from occipital and temporal leads in the two cerebral hemispheres of eight fencers and eight control subjects. The stimulus was a checkerboard subtending a small (1 degree) or large (30 degrees) visual field. Significant differences in P60, N75 and P100 latency and amplitude were found between the two subject groups, especially during the processing of the large visual field. In fencers and left-handers shorter latencies were found for the large visual field condition, whereas right-handers showed an opposite trend. The results give further evidence of special patterns of visual processing in athletes, like fencers, in agreement with the literature.

Visual evoked potentials (VEPs) were recorded in 17 females (10 right-handers and 7 left-handers) and 17 males (10 right-handers and 7 left-handers). The stimulus was a checkerboard pattern phase-reversed at a rate of 1 Hz, binocularly viewed by the subject. Two experimental conditions were tested: field size (the stimulus was enlarged from 1 to 30 degrees of visual angle) and occlusion (the central part of the stimulus was occluded by experimental scotomata of various size--from 1 to 16 degrees). Responses recorded at occipital and temporal leads of the two hemispheres were compared. The P100-N145 amplitude was larger at occipital than at temporal leads, it augmented increasing the size of the stimulus and diminished occluding progressively larger portions of the central field. Hemispheric asymmetries emerged with large field sizes and were not abolished by the presence of experimental scotomata. The hemispheric asymmetry was related to handedness (left-handers had larger amplitudes in the right hemisphere, right-handers in the left hemisphere), but not to sex. Hemispheric asymmetries in VEPs latency were not present. In comparison to males, females showed larger amplitudes and shorter latencies. These results point out that hemispheric asymmetries are found also for the processing of elementary visual stimuli like checkerboards, and they depend both on stimulus parameters (field size) and subject variables (handedness).