EVOKED RESPONSES IN RELATION TO VISUAL PERCEPTION AND OCULOMOTOR REACTION TIMES IN MAN

Spatial Attention and Temporal Expectation Under Timed Uncertainty Predictably Modulate Neuronal Responses in Monkey V1

The brain uses attention and expectation as flexible devices for optimizing behavioral responses associated with expected but unpredictably timed events. The neural bases of attention and expectation are thought to engage higher cognitive loci; however, their influence at the level of primary visual cortex (V1) remains unknown. Here, we asked whether single-neuron responses in monkey V1 were influenced by an attention task of unpredictable duration. Monkeys covertly attended to a spot that remained unchanged for a fixed period and then abruptly disappeared at variable times, prompting a lever release for reward. We show that monkeys responded progressively faster and performed better as the trial duration increased. Neural responses also followed monkey's task engagement-there was an early, but short duration, response facilitation, followed by a late but sustained increase during the time monkeys expected the attention spot to disappear. This late attentional modulation was significantly and negatively correlated with the reaction time and was well explained by a modified hazard function. Such bimodal, time-dependent changes were, however, absent in a task that did not require explicit attentional engagement. Thus, V1 neurons carry reliable signals of attention and temporal expectation that correlate with predictable influences on monkeys' behavioral responses.

Visually evoked potentials evoked by moving unidimensional noise stimuli: effects of contrast, spatial frequency, active electrode location, reference electrode location, and stimulus type

We determined the relative importance of electrode derivation, stimulus type, spatial frequency and contrast in determining the relative size of the late negative and early positive responses of motion elicited VEPs. Seven subjects aged 22-48 years with normal vision were tested binocularly. Motion onset and motion reversal were employed as modes of stimulus presentation. For both, pseudo-random one-dimensional noise patterns whose peak power was at 5.2, 2.6, 1.3, 0.325 and 0.1625 cycles per degree (cpd) were stimuli. Contrasts were 70% and 5%. Active electrodes were placed at Oz, 5 cm to the left of Oz, 5 cm to the right of Oz and a frontal midline position (Fpz) and referenced to linked mastoids. Transient motion reversal elicited a prominent positive response present in all subjects and at low contrasts. Motion onset VEPs have a complex waveform which may be either predominantly positive or negative. The most important variables in determining whether a prominent positivity or negativity is present in the motion onset VEP are the contrast and the spatial frequencies. Data such as these are first efforts in developing recommendations for the motion VEP.

Visual evoked potentials elicited by a moving unidimensional noise pattern

(1) Motion onset and offset visual evoked potentials (VEPs) were recorded in normal human subjects using a unidimensional noise pattern moving at 1, 8 and 64 degrees/s. The maximum N1-P1 amplitude of the motion onset response was obtained when using a fine noise pattern (maximum energy at 5.2 cpd) moving at 8 degrees/s. (2) At a velocity of 8 degrees/s, the motion onset response (fine pattern, 0.70 contrast) showed a morphology similar to the pattern disappearance response. Both at a lower (1 degrees/s) and a higher velocity (64 degrees/s) the N1-P1 amplitude of the motion onset complex was significantly reduced. The latency of the motion onset response (8 degrees/s) and the pattern disappearance complex were significantly different. (3) The effect of lowering the spatial content of the noise pattern on the amplitude of the motion onset response was different for the 3 velocities tested: the largest effect was at the lower velocity of 1 degrees/s; there was no similar effect on the pattern disappearance response. (4) With decreasing contrast, the N1-P1 amplitude of the motion onset response at 8 degrees/s decreased, but this reduction in amplitude was much less than that of the disappearance response. The contrast dependency of the motion onset complex was identical for binocular and monocular recordings. (5) Increasing the motion duration or the duration of the interstimulus interval did not alter the general morphology of the motion response.

Visual evoked potentials under varied velocities of continuous and discrete apparent motion

This experiment was undertaken to investigate the effects of different types and velocities of apparent motion on the Visual Evoked Potential (VEP). Central (Cz) recording site VEPs were obtained from four males and three females under seven conditions. Significant differences in terms of amplitude and latencies were found between continuous and discrete apparent motion at the Oz derivation only. Theoretical and cell system models describing separate cortical channels for processing continuous and discrete apparent motion were presented to account for the observed differences. The stationary condition produced a significantly shorter latency than all other conditions at Cz. In addition, discernable trends observed between central and occipital recording sites suggest the role of the occipital cortex as the primary processor of motion and for detecting differences in motion velocity.

Hemispheric sex differences in response to apparently moving stimuli as indicated by visual evoked potentials

This research was designed to determine whether visual event related potentials (ERPs) recorded from over the two occipital hemispheres would indicate differential processing of apparently moving and stationary stimuli. Previous findings in the literature led us to consider a male-female comparison as part of our experimental design. Nine male and nine female subjects were screened for right-handedness and visual abilities (acuity, depth and phoria). Each participated in three experimental sessions over a three day period in which ERPs were recorded from O1 and O2 in response to stimuli presented in left, central and right visual fields. The main finding was that, for female subjects, right hemispheric derived ERPs were larger in amplitude than left hemisphere recordings for apparently moving stimuli presented centrally. Males showed no hemispheric amplitude differences. However, for male subjects, the left hemisphere ERP latencies to motion were longer than those recorded from over O2 with central visual field presentations. It was speculated that different attentional strategies in the two sexes led to the observed result.