Compelling classroom demonstrations that link visual system anatomy, physiology, and behaviour.
(17/121)
One of our approaches to teaching a course in anatomy and physiology is to stress the fundamental, systems-level concepts. One successful strategy we use is to continually highlight the relationships among anatomy, physiology, and behavior. In this article, we describe a set of classroom demonstrations that stress these links while fostering critical thinking. These demonstrations, on the topic of sensory system structure and function, rely on two perceptual consequences of neural adaptation in the visual system: afterimages and aftereffects. Viewing specific visual stimuli under binocular or monocular conditions with interocular transfer permits several concepts to be observed and discussed, including neural adaptation, anatomical and functional segregation of visual system pathways, and the relationship among visual system structure, function, and perception. This article discusses how to produce and present the required visual stimuli, suggests a set of questions to stimulate critical thinking, and presents student evaluation of this activity. (+info)
A functional angle on some after-effects in cortical vision.
(18/121)
The question of how our brains and those of other animals code sensory information is of fundamental importance to neuroscience research. Visual illusions offer valuable insight into the mechanisms of perceptual coding. One such illusion, the tilt after-effect (TAE), has been studied extensively since the 1930s, yet a full explanation of the effect has remained elusive. Here, we put forward an explanation of the TAE in terms of a functional role for adaptation in the visual cortex. The proposed model accounts not only for the phenomenology of the TAE, but also for spatial interactions in perceived tilt and the effects of adaptation on the perception of direction of motion and colour. We discuss the implications of the model for understanding the effects of adaptation and surround stimulation on the response properties of cortical neurons. (+info)
Masking, persistence, and transfer in rotating arcs.
(19/121)
We demonstrate that the apparent length of a thin white arc on a black disk, rotating concentrically at 2.5 rps, varies with angular length and exposure duration. While short arcs (9-18 degrees ) gradually expand, long arcs (36-72 degrees ) first undergo a brief contraction, before they also expand. On average, perceived elongation asymptotes after 15 s equivalent to visual persistencies ranging from 68 to 170 ms. Using bi- and tri-colored arcs, we find that the apparent increase in length derives from the rear end of the rotating stimulus, while the initial shrinkage derives from contraction of the middle. After 15 s of adaptation, perceived length of the arc decays to actual stimulus length within an average of 6 s and, upon re-exposure of the arc, reaches its former value after only 5 s (priming). When the rotating arc is presented first to one eye and then to the other, apparent elongation transfers partially (46%), suggesting a contribution by the binocular cells in the visual cortex. A partial transfer (26%) also occurs from clockwise to counterclockwise rotation. When tested interocularly, the directional transfer is more pronounced (47%) and equals the interocular transfer under equidirectional conditions, suggesting that the directional transfer (cw versus ccw) might derive from non-directional cortical units. Whereas the initial contraction may be attributable to backward masking, the observed elongation likely reflects a cumulative build-up of after-discharge in cortical neurons over time. (+info)
Nulling the motion aftereffect with dynamic random-dot stimuli: limitations and implications.
(20/121)
We used biased random-dot dynamic test stimuli to measure the strength of the motion aftereffect (MAE) to evaluate the usefulness of this technique as a measure of motion adaptation strength. The stimuli consisted of noise dots whose individual directions were random and of signal dots moving in a unique direction. All dots moved at the same speed. For each condition, the nulling percentage (percentage of signal dots needed to perceptually null the MAE) was scaled with respect to the coherence threshold (percentage needed to perceive the coherent motion of signal dots without prior adaptation). The increase of these scaled values with the density of dots in the test stimulus suggests that MAE strength is underestimated when measured with low densities. We show that previous reports of high nulling percentages at slow speeds do not reflect strong MAEs, but are actually due to spatio-temporal aliasing, which dramatically increases coherence thresholds. We further show that MAE strength at slow speed increases with eccentricity. These findings are consistent with the idea that using this dynamic test stimulus preferentially reveals the adaptation of a population of high-speed motion units whose activity is independent of adapted low-speed motion units. (+info)
Pupil responses associated with coloured afterimages are mediated by the magno-cellular pathway.
(21/121)
Sustained fixation of a bright coloured stimulus will, on extinction of the stimulus and continued steady fixation, induce an afterimage whose colour is complementary to that of the initial stimulus; an effect thought to be caused by fatigue of cones and/or of cone-opponent processes to different colours. However, to date, very little is known about the specific pathway that causes the coloured afterimage. Using isoluminant coloured stimuli recent studies have shown that pupil constriction is induced by onset and offset of the stimulus, the latter being attributed specifically to the subsequent emergence of the coloured afterimage. The aim of the study was to investigate how the offset pupillary constriction is generated in terms of input signals from discrete functional elements of the magno- and/or parvo-cellular pathways, which are known principally to convey, respectively, luminance and colour signals. Changes in pupil size were monitored continuously by digital analysis of an infra-red image of the pupil while observers viewed isoluminant green pulsed, ramped or luminance masked stimuli presented on a computer monitor. It was found that the amplitude of the offset pupillary constriction decreases when a pulsed stimulus is replaced by a temporally ramped stimulus and is eliminated by a luminance mask. These findings indicate for the first time that pupillary constriction associated with a coloured afterimage is mediated by the magno-cellular pathway. (+info)
Timing of contextual modulation in the shine-through effect.
(22/121)
Contextual elements can fundamentally change the perception of an embedded target. A recently discovered masking effect, shine-through, allows one to investigate the precise dynamics of contextual modulation of the human visual system. In this shine-through effect, a vernier precedes a grating comprising more than seven elements for display times as short as 10 ms. The vernier appears as a "shine-through" element superimposed on the grating. However, if additional single lines are presented above and below the grating, visibility of the shine-through element dramatically diminishes. Recent publications focused mainly on the spatial aspects of this contextual modulation. Here, we investigate its temporal characteristics. We show that contextual suppression can occur for context durations of 5-10 ms, even if contextual elements appear 100 ms before target onset. This contextual suppression is not due to the presentation of the contextual elements themselves since without the grating contextual elements exert only weak masking power. Only the combination of contextual elements and grating causes the contextual suppression. (+info)
Brain potentials associated with conscious aftereffects induced by unseen stimuli in a blindsight subject.
(23/121)
The study is of brain activity in a blindsight subject (D.B.), who reports conscious visual afterimages of stimuli of which he is unaware when they are presented. This contrast offered a unique opportunity to study event-related potential recordings of conscious versus unconscious visual phenomena generated by the very same stimulus in the identical locus of the visual field. The behavioral results confirmed the reliability of the difference in the subject's report for inducing stimuli versus their aftereffects. The rationale of the event-related potential analysis was to subtract "on" signals from "off" signals, the latter associated with the onset of conscious events and the former for events that remained unconscious. Because there are inherent differences in on and off potentials, the subtractive resultants for the blind hemifield were compared with the same subtractions for the good hemifield when the subject was aware both of the stimuli and their afterimages. A differential pattern in subtractive resultants emerged with a strong anterior left frontal focus for the blind field and a posterior focus for the intact field. The results are compared with other studies suggesting an anterior focus for conscious visual events. (+info)
Extending the shine-through effect to classical masking paradigms.
(24/121)
A vernier, presented for a short time, shines through a following grating if the grating contains nine and more elements but remains largely invisible for smaller gratings. Therefore, extended grating masks yield, surprisingly, less masking than smaller ones. Here, we show that this mask size effect is not unique to grating masks. Masking diminishes if the size of classical pattern-, noise-, light-, and metacontrast masks increases and if these masks are regular, i.e. highly ordered. (+info)