Biomedical subjects

S L Macknik

Publications and source records attributed to S L Macknik.

7 recordsLinked to original sources

PubMed2000

The role of spatiotemporal edges in visibility and visual masking.

What parts of a visual stimulus produce the greatest neural signal? Previous studies have explored this question and found that the onset of a stimulus's edge is what excites early visual neurons most strongly. The role of inhibition at the edges of stimuli has remained less clear, however, and the importance of neural responses associated with the termination of stimuli has only recently been examined. Understanding all of these spatiotemporal parameters (the excitation and inhibition evoked by the stimulus's onset and termination, as well as its spatial edges) is crucial if we are to develop a general principle concerning the relationship between neural signals and the parts of the stimulus that generate them. Here, we use visual masking illusions to explore this issue, in combination with human psychophysics, awake behaving primate neurophysiology in the lateral geniculate nucleus of the thalamus, and optical recording in the primary visual cortex of anesthetized monkeys. The edges of the stimulus, rather than its interior, generate the strongest excitatory and inhibitory responses both perceptually and physiologically. These edges can be imaged directly by using optical recording techniques. Excitation and inhibition are moreover most powerful when the stimulus turns both on and off (what might be thought of as the stimulus's temporal edges). We thus conclude that there is a general principle that relates the generation of neural signals (excitatory and inhibitory) to the spatiotemporal edges of stimuli in the early visual system.

Animals↗

PubMed2000

Microsaccadic eye movements and firing of single cells in the striate cortex of macaque monkeys.

When viewing a stationary object, we unconsciously make small, involuntary eye movements or 'microsaccades'. If displacements of the retinal image are prevented, the image quickly fades from perception. To understand how microsaccades sustain perception, we studied their relationship to the firing of cells in primary visual cortex (V1). We tracked eye movements and recorded from V1 cells as macaque monkeys fixated. When an optimally oriented line was centered over a cell's receptive field, activity increased after microsaccades. Moreover, microsaccades were better correlated with bursts of spikes than with either single spikes or instantaneous firing rate. These findings may help explain maintenance of perception during normal visual fixation.

Action Potentials↗

PubMed1999

Optical images of visible and invisible percepts in the primary visual cortex of primates.

We optically imaged a visual masking illusion in primary visual cortex (area V-1) of rhesus monkeys to ask whether activity in the early visual system more closely reflects the physical stimulus or the generated percept. Visual illusions can be a powerful way to address this question because they have the benefit of dissociating the stimulus from perception. We used an illusion in which a flickering target (a bar oriented in visual space) is rendered invisible by two counter-phase flickering bars, called masks, which flank and abut the target. The target and masks, when shown separately, each generated correlated activity on the surface of the cortex. During the illusory condition, however, optical signals generated in the cortex by the target disappeared although the image of the masks persisted. The optical image thus was correlated with perception but not with the physical stimulus.

Animals↗

PubMed1999

Wakes and spokes: new motion-induced brightness illusions.

Under certain conditions, high-contrast moving figures induce adjacent illusory regions, 'wakes' and 'spokes', which have contrast polarity opposite the inducing figures. In this paper we document properties of these novel phenomena. When the illusions are induced by a moving bar, spokes appear on the side of the bar closer to fixation and connect the bar to the fixation point, regardless of the momentary position of the bar or whether it is moving to the left or to the right. Although spokes often extend up to the fixation point, they never extend beyond it. This is not due to blocking of the spoke's spread by the fixation point, because in another experiment spokes extend directly through an intervening figure. Whereas spokes emanate from the end of a horizontally moving bar closest to fixation, wakes emanate from the end farthest from fixation. In contrast to spokes, wakes do not show a towards-fixation bias. Instead, the wake's end trails the position of the bar, like a ship's wake. The higher the bar velocity, the more the end of the wake appears to trail it, suggesting that wakes are caused by a process which spreads from the edge of moving figures. Wakes and spokes, as distinct illusions, should provide significant constraints on theories of human motion and brightness perception processes.

Contrast Sensitivity↗

PubMed1998

Neuronal correlates of visibility and invisibility in the primate visual system.

A brief visual target stimulus may be rendered invisible if it is immediately preceded or followed by another stimulus. This class of illusions, known as visual masking, may allow insights into the neural mechanisms that underlie visual perception. We have therefore explored the temporal characteristics of masking illusions in humans, and compared them with corresponding neuronal responses in the primary visual cortex of awake and anesthetized monkeys. Stimulus parameters that in humans produce forward masking (in which the mask precedes the target) suppress the transient on-response to the target in monkey visual cortex. Those that produce backward masking (in which the mask comes after the target) inhibit the transient after-discharge, the excitatory response that occurs just after the disappearance of the target. These results suggest that, for targets that can be masked (those of short duration), the transient neuronal responses associated with onset and turning off of the target may be important in its visibility.

Animals↗

PubMed1995

Saccadic suppression relies on luminance information.

To determine whether saccadic suppression of image displacement uses information from luminance channels, we measured spatial displacement detection thresholds with equiluminant and non-equiluminant targets during saccades. We compared these saccadic thresholds with displacement thresholds measured during fixation by making ratios of saccadic thresholds to fixation thresholds. Ratios were lower in the equiluminant condition than in the non-equiluminant. This surprising result indicates that detection of equiluminant target displacements during saccades was better than detection of nonequiluminant targets, compared with the detection abilities during fixation. Thus, saccadic suppression of image displacement, which should increase displacement thresholds during saccades over fixation thresholds, was more effective with nonequiluminant targets. Because of target flicker, displacement thresholds were anisotropic in the nonequiluminant condition; thresholds were greater when target and eye moved in the same direction than when they moved in opposite directions, consistent with earlier results. These two effects (flicker-induced anisotropy and greater suppression in nonequiluminance) canceled when the eye moved opposite the displacement, yielding equal thresholds and summed when eye and target moved in the same direction, yielding large threshold differences. We conclude that saccadic suppression of image displacement uses mechanisms sensitive to luminance contrast.

Anisotropy↗

PubMed1991

Flicker distorts visual space constancy.

Effects of flicker on space perception were measured by displacing a flickering target during saccadic eye movements. A small target was flickered at 33, 66, 130 or 260 Hz. Using a 2-interval forced-choice design, sensitivity to the displacement was about twice as great when the target was moved in the direction opposite the eye movement as when it was moved in the same direction. This would be expected from a partial breakdown of space constancy--the world should seem to jump in the direction opposite an eye movement. Even if a suppression of displacement detection during saccades prevents this jump from being perceived; it should be easier to detect a target displacement in the direction opposite the eye movement than in the same direction: when movement is opposite, the imposed displacement adds to the illusory displacement, making detection easier. Displacements were more easily detected at lower flicker rates. Results imply that both masking and extraretinal signals are important in suppressing the detectability of target displacements during saccades, and that flicker on video display terminals may distort space perception.

Humans↗