Simulated Electrophysiology Experiments Provide Previously Inaccessible Insights into Visual Physiology.

Lecture content and practical laboratory classes are ideally complementary. However, the types of experiments that have led to our detailed understanding of sensory neuroscience are often not amenable to classroom experimentation as they require expensive equipment, time-consuming surgeries, specialized experimental techniques, and the use of animals. While sometimes feasible in small group teaching, these experiments are not suitable for large cohorts of students.

Orientation selectivity in rat primary visual cortex emerges earlier with low-contrast and high-luminance stimuli.

In natural vision, rapid and sustained variations in luminance and contrast change the reliability of information available about a visual scene, and markedly affect both neuronal and behavioural responses. The hallmark property of neurons in primary visual cortex (V1), orientation selectivity, is unaffected by changes in stimulus contrast, but it remains unclear how sustained differences in mean luminance and contrast affect the time-course of orientation selectivity, and the amount of information that neurons carry about orientation. We used reverse correlation with characterize the temporal dynamics of orientation selectivity in rat V1 neurons under four luminance-contrast conditions.

Masking reduces orientation selectivity in rat visual cortex.

In visual masking the perception of a target stimulus is impaired by a preceding (forward) or succeeding (backward) mask stimulus. The illusion is of interest because it allows uncoupling of the physical stimulus, its neuronal representation, and its perception. To understand the neuronal correlates of masking, we examined how masks affected the neuronal responses to oriented target stimuli in the primary visual cortex (V1) of anesthetized rats (n = 37).

Rapid Adaptation Induces Persistent Biases in Population Codes for Visual Motion.

Unlabelled: Each visual experience changes the neural response to subsequent stimuli. If the brain is unable to incorporate these encoding changes, the decoding, or perception, of subsequent stimuli is biased. Although the phenomenon of adaptation pervades the nervous system, its effects have been studied mainly in isolation, based on neuronal encoding changes induced by an isolated, prolonged stimulus.

Noisy decision thresholds can account for suboptimal detection of low coherence motion.

Noise in sensory signals can vary over both space and time. Moving random dot stimuli are commonly used to quantify how the visual system accounts for spatial noise. In these stimuli, a fixed proportion of "signal" dots move in the same direction and the remaining "noise" dots are randomly replotted.

The (un)suitability of modern liquid crystal displays (LCDs) for vision research.

Psychophysical and physiological studies of vision have traditionally used cathode ray tube (CRT) monitors to present stimuli. These monitors are no longer easily available, and liquid crystal display (LCD) technology is continually improving; therefore, we characterized a number of LCD monitors to determine if newer models are suitable replacements for CRTs in the laboratory. We compared the spatial and temporal characteristics of a CRT with five LCDs, including monitors designed with vision science in mind (ViewPixx and Display++), "prosumer" gaming monitors, and a consumer-grade LCD.

Reflexive tracking eye movements and motion perception: one or two neural populations?

Motion-sensitive neurons in the middle temporal (MT) and medial superior temporal (MST) areas perform the sensory analysis required for both motion perception and controlling smooth eye movements. The perceptual and oculomotor systems are characterized by high variability, even when responding to identical stimulus repetitions. If a single population of neurons performs the motion analysis driving perception and eye movements, errors in perception and action might show similar direction-dependent biases, or their variability might be correlated across trials.

Adaptation to speed in macaque middle temporal and medial superior temporal areas.

The response of a sensory neuron to an unchanging stimulus typically adapts, showing decreases in response gain that are accompanied by changes in the shape of tuning curves. It remains unclear whether these changes arise purely due to spike rate adaptation within single neurons or whether they are dependent on network interactions between neurons. Further, it is unclear how the timescales of neural and perceptual adaptation are related.

Accurate reading with sequential presentation of single letters.

Rapid, accurate reading is possible when isolated, single words from a sentence are sequentially presented at a fixed spatial location. We investigated if reading of words and sentences is possible when single letters are rapidly presented at the fovea under user-controlled or automatically controlled rates. When tested with complete sentences, trained participants achieved reading rates of over 60 wpm and accuracies of over 90% with the single letter reading (SLR) method and naive participants achieved average reading rates over 30 wpm with greater than 90% accuracy.

Adaptation to direction statistics modulates perceptual discrimination.

Perception depends on the relative activity of populations of sensory neurons with a range of tunings and response gains. Each neuron's tuning and gain are malleable and can be modified by sustained exposure to an adapting stimulus. Here, we used a combination of human psychophysical testing and models of neuronal population decoding to assess how rapid adaptation to moving stimuli might change neuronal tuning and thereby modulate direction perception.

Timescales of sensory- and decision-related activity in the middle temporal and medial superior temporal areas.

The contribution of sensory neurons to perceptual decisions about external stimulus events has received much attention, but it is less clear how sensory responses are integrated over time to produce decisions that are both rapid and reliable. To address this issue, we recorded from middle temporal area and medial superior temporal area neurons in rhesus macaques performing a task requiring the detection and discrimination of unpredictable speed changes. We examined how neuronal activity encoded the sign of the speed change and predicted the animals' behavioral judgments and reaction times, with a focus on the timescales over which neuronal activity is informative.

Direction and contrast tuning of macaque MSTd neurons during saccades.

Saccades are rapid eye movements that change the direction of gaze, although the full-field image motion associated with these movements is rarely perceived. The attenuation of visual perception during saccades is referred to as saccadic suppression. The mechanisms that produce saccadic suppression are not well understood.

Saccadic modulation of neural responses: possible roles in saccadic suppression, enhancement, and time compression.

Humans use saccadic eye movements to make frequent gaze changes, yet the associated full-field image motion is not perceived. The theory of saccadic suppression has been proposed to account for this phenomenon, but it is not clear whether suppression originates from a retinal signal at saccade onset or from the brain before saccade onset. Perceptually, visual sensitivity is reduced before saccades and enhanced afterward.

Characterizing contrast adaptation in a population of cat primary visual cortical neurons using Fisher information.

When cat V1/V2 cells are adapted to contrast at their optimal orientation, a reduction in gain and/or a shift in the contrast response function is found. We investigated how these factors combine at the population level to affect the accuracy for detecting variations in contrast. Using the contrast response function parameters from a physiologically measured population, we model the population accuracy (using Fisher information) for contrast discrimination.

Tuning properties of radial phantom motion aftereffects.

Motion aftereffects are normally tested in regions of the visual field that have been directly exposed to motion (local or concrete MAEs). We compared concrete MAEs with remote or phantom MAEs, in which motion is perceived in regions not previously adapted to motion. Our aim was to study the spatial dependencies and spatiotemporal tuning of phantom MAEs generated by radially expanding stimuli.

Direction-selective neurons in the optokinetic system with long-lasting after-responses.

We describe the responses during and after motion of slow cells, which are a class of direction-selective neurons in the pretectal nucleus of the optic tract (NOT) of the wallaby. Neurons in the NOT respond to optic flow generated by head movements and drive compensatory optokinetic eye movements. Motion in the preferred direction produces increased firing rates in the cells, whereas motion in the opposite direction inhibits their high spontaneous activities.