Biomedical subjects

Michael A Paradiso

Publications and source records attributed to Michael A Paradiso.

7 recordsLinked to original sources

PubMed2006

Stimulus for rapid ocular dominance plasticity in visual cortex.

Although it has been known for decades that monocular deprivation shifts ocular dominance in kitten striate cortex, uncertainty persists about the adequate stimulus for deprivation-induced losses of cortical responsiveness. In the current study we compared the effects of 2 days of lid closure and 2 days of monocular blur using an overcorrecting contact lens. Our finding of comparable ocular dominance shifts in visual cortex indicates that deprived-eye response depression is not a result of reduced retinal illumination. The quality rather than the quantity of retinal illumination is the key factor for ocular dominance plasticity. These data have implications for both the mechanism and treatment of amblyopia.

Action Potentials↗

PubMed2006

Lightness, filling-in, and the fundamental role of context in visual perception.

Visual perception is defined by the unique spatial interactions that distinguish it from the point-to-point precision of a photometer. Over several decades, Lothar Spillmann has made key observations about the nature of these interactions and the role of context in perception. Our lab has explored the perceptual properties of spatial interactions and more generally the importance of visual context for neuronal responses and perception. Our investigations into the spatiotemporal dynamics of lightness provide insight into underlying mechanisms. For example, backward masking and luminance modulation experiments suggest that the representation of a uniformly luminous object develops first at the borders and, in some manner, the center fills in. The temporal dynamics of lightness induction are also consistent with a filling-in process. There is a slow cutoff temporal frequency above which surround luminance modulation will not elicit perceptual induction of a central area. The larger the central area, the lower the cutoff frequency for induction, perhaps indicating that an edge-based process requires more time to "complete" the larger area. In recordings from primary visual cortex we find that neurons respond in a manner surprisingly consistent with lightness perception and the spatial and temporal properties of induction. For example, the activity of V1 neurons can be modulated by light outside the receptive field and as the modulation rate is increased response modulation falls off more rapidly for large uniform areas than smaller areas. The conclusion we draw from these experiments is that lightness appears to be computed slowly on the basis of edge and context information. A possible role for the spatial interactions is lightness constancy, which is thought to depend on extensive spatial integration. We find not only that V1 responses are strongly context dependent, but that this dependence makes V1 lightness constant on average. The dependence of constancy on surround interactions underscores the fundamental role that context plays in perception. In more recent studies, further support has been found for the importance of context in experiments using natural scene stimuli.

Animals↗

PubMed2005

Background changes delay information represented in macaque V1 neurons.

In natural behavioral situations, saccadic eye movements not only introduce new stimuli into V1 receptive fields, they also cause changes in the background. We recorded in awake macaque V1 using a fixation paradigm and compared evoked activity to small stimuli when the background was either static or changing as with a saccade. When a stimulus was shown on a static background, as in most previous experiments, the initial response was orientation selective and contrast was inversely correlated with response latency. When a stimulus was introduced with a background change, V1 neurons showed a qualitatively different temporal response pattern in which information about stimulus orientation and contrast was delayed. The delay in the representation of visual information was found with three different types of background change-luminance increment, luminance decrement, and a pattern change with fixed mean luminance. We also found that with a background change, V1 off responses were suppressed and had a shorter time course compared with the static-background situation. Our results suggest that the distribution of temporal changes across the visual field plays a fundamental role in determining V1 responses. In the static-background condition, temporal change in the visual input occurs only in a small portion of the visual field. In the changing-background condition, and presumably in natural vision, temporal changes are widely distributed. Thus a delayed representation of visual information may be more representative of natural visual situations.

Action Potentials↗

PubMed2005

Background changes delay the perceptual availability of form information.

In natural visual situations, unlike most psychophysical experiments, when a new stimulus appears in a portion of the visual field, the surrounding background changes simultaneously. In recordings from macaque V1, we found that a visual stimulus presented simultaneously with a background change evokes a response that is qualitatively different from the response to the same stimulus flashed on a static background. With the changing background, information about stimulus orientation and contrast is significantly delayed compared with the static-background situation. Our physiological results make several predictions that we test in the present paper with human psychophysical experiments. In a backward masking paradigm, a bar stimulus was either flashed onto a static background or presented simultaneously with a change in background luminance or pattern. Subjects discriminated bar orientation or detected that the scene changed before the mask. To achieve an equivalent contrast threshold for orientation discrimination, a longer stimulus-mask onset asynchrony (SOA) was needed in the changing than in the static-background condition; to match the orientation discrimination performance in the static and changing-background conditions at a fixed SOA, a higher bar contrast was needed when the background changed. Moreover, in the changing-background condition, a longer SOA was needed to discriminate bar orientation than to detect the scene change. These results suggest that orientation information is available more slowly when the background changes; orientation information is available earlier as stimulus contrast increases. The psychophysical findings are consistent with our physiological predictions. Compared with the common technique of flashing stimuli onto a static background, the changing-background paradigm may be more similar to natural vision in which saccades bring new stimuli and backgrounds into the visual field.

Contrast Sensitivity↗

PubMed2005

The importance of modulatory input for V1 activity and perception.

To conduct well-controlled studies of visual processing in the laboratory, deviations from natural visual situations must generally be employed. In some regards, the reduced visual paradigms typically used are adequate for providing an accurate description of visual representations. However, the use of fixation paradigms and stimuli isolated within a receptive field may underestimate the richness of visual processing in area V1. Experiments ranging from lightness encoding and perception to paradigms involving natural scenes and saccades used to examine the relationship between V1 activity and perception are reviewed in this chapter. Using more complex and natural visual stimulation, V1 responses have been detected that are significantly different from responses obtained in more reduced paradigms. A feature common to the findings of different experiments is that the scale of the activated neural population and circuitry appears to play a key role in the correlation between V1 activity and perception. More complex and natural visual stimulation brings into play extra-receptive field modulatory input not involved with stimulation localized to the receptive field. The results suggest that rather than subtly sculpting the response, modulatory input coming from intra- and/or intercortical sources is fundamental in establishing perceptual response patterns in natural visual situations.

Animals↗

PubMed2002

Perception of brightness and brightness illusions in the macaque monkey.

Recent physiological studies show that neural responses correlated with the perception of brightness are found in cortical area V1 but not earlier in the visual pathway (Kayama et al., 1979; Reid and Shapley, 1989; Squatrito et al., 1990; Komatsu et al., 1996; Rossi et al., 1996; MacEvoy et al., 1998; Rossi and Paradiso, 1999; Hung et al., 2001; Kinoshita and Komatsu, 2001; MacEvoy and Paradiso, 2001). However, these studies are based on comparisons of neural responses in animals with brightness perception in humans. Very little is known about the perception of brightness in animals typically used in physiological experiments. In this study, we quantify brightness discrimination, brightness induction, and White's effect in macaque monkeys. The results show that, qualitatively and quantitatively, the perception of brightness in macaques and humans is quite similar. This similarity may be an indication of common underlying neural computations in the two species.

Animals↗

PubMed2002

Perceptual and neuronal correspondence in primary visual cortex.

Recent findings from the study of primary visual cortex in humans and animals blur the distinction between early and late visual processing. Under some conditions, the activity of neurons in primary visual cortex appears as close or closer to perception than activity in 'higher' visual areas.

Animals↗