1,720,969 research outputs found
Spatial Organization of Receptive Fields of V1 Neurons of Alert Monkeys: Comparison With Responses to Gratings
No full textWe studied the spatial organization of receptive fields and the responses to gratings of neurons in parafoveal V1 of alert monkeys. Activating regions (ARs) of 228 cells were mapped with increment and decrement bars while compensating for fixational eye movements. For cells with two or more ARs, the overlap between ARs responsive to increments (INC) and ARs responsive to decrements (DEC) was characterized by a quantitative overlap index (OI). The distribution of overlap indices was bimodal. The larger group (78% of cells) was composed of complex cells with strongly overlapping ARs (OI >/= 0.5). The smaller group (14%) was composed of simple cells with minimal spatial overlap of ARs (OI 1, the traditional criterion for identifying simple cells. However, unlike simple cells, even those complex cells with high RM could exhibit diverse nonlinear responses when the spatial frequency or window size was changed. Furthermore, the responses of complex cells to counterphase gratings were predominantly nonlinear even harmonics. These results show that RM is not a robust test of linearity. Our results indicate that complex cells are the most frequently encountered neurons in primate V1, and their behavior needs to receive more emphasis in models of visual function
Reply to: The linearity and selectivity of neuronal responses in awake visual cortex, Chen et al. (2009)
No full textSaccades and drifts differentially modulate neuronal activity in V1: Effects of retinal image motion, position, and extraretinal influences
No full textIn natural vision, continuously changing input is generated by fast saccadic eye movements and slow drifts. We analyzed effects of fixational saccades, voluntary saccades, and drifts on the activity of macaque V1 neurons. Effects of fixational saccades and small voluntary saccades were equivalent. In the presence of a near-optimal stimulus, separate populations of neurons fired transient bursts after saccades, sustained discharges during drifts, or both. Strength, time course, and selectivity of activation by fast and slow eye movements were strongly correlated with responses to flashed or to externally moved stimuli. These neuronal properties support complementary functions for post-saccadic bursts and drift responses. Local post-saccadic bursts signal rapid motion or abrupt change of potentially salient stimuli within the receptive field; widespread synchronized bursts signal occurrence of a saccade. Sustained firing during drifts conveys more specific information about location and contrast of small spatial features that contribute to perception of fine detail. In addition to stimulus-driven responses, biphasic extraretinal modulation accompanying saccades was identified in one third of the cells. Brief perisaccadic suppression was followed by stronger and longer-lasting enhancement that could bias perception in favor of saccade targets. These diverse patterns of neuronal activation underlie the dynamic encoding of our visual world
Lack of short-term adaptation in V1 cells of the alert monkey
No full textAlthough prolonged adaptation is a very useful experimental tool, its relevancy to normal vision, which consists of series of short fixational epochs, is not clear. Directly related to perception is short-term pattern adaptation where a stimulus is quickly followed by an identical one. Recently, strong adaptation effects have been demonstrated in both V1 single cell recordings and fMRI. V1 single cells adaptation has been proposed as the basis of fMRI adaptation and as improving the discriminability of signals arising from successively viewed images of similar structures. We have previously shown, in alert monkeys, that responses of V1 cells to repeated stimuli is much less variable than that found in the anesthetized preparation. Here we report on the degree of adaptation in V1 cells to repeated, identical brief stimuli. Recordings were made from single cells in area V1 of alert monkeys performing a fixation task. The cells' spatial organization was studied with drifting increment and decrement bars while compensating for fixational drifts. Cells were stimulated with optimally oriented flashing or sweeping bars during a 5 sec fixation trial. No cells showed the strong clear adaptation effects found in the anesthetized preparation and the great majority did not adapt at all. When non-optimal stimuli were used, response variability increased. We conclude that fMRI adaptation does not originate in V1 and that, in repeated viewing, discriminability between similar but not identical features is predicated on different responses of neuronal populations finely tuned to different features. That responses to optimal stimuli do not adapt and are much less variable than responses to non-optimal ones, is consistent with this discrimination scheme
Primate area V1: largest response gain for receptive fields in the straight-ahead direction
No full textAlthough neuronal responses in behaving monkeys are typically studied while the monkey fixates straight ahead, it is known that eye position modulates responses of visual neurons. The modulation has been found to enhance neuronal responses when the receptive field is placed in the straight-ahead position for neurons receiving input from the peripheral but not the central retina. We studied the effect of eye position on the responses of V1 complex cells receiving input from the central retina (1.1–5.7° eccentricity) while minimizing the effect of fixational eye movements. Contrast response functions were obtained separately with drifting light and dark bars. Data were fit with the Naka–Rushton equation: r(c)=Rmax×cn/(cn+c50n)+s, where r(c) is mean spike rate at contrast c, Rmax is the maximum response, c50 is the contrast that elicits half of Rmax, and s is the spontaneous activity. Contrast sensitivity as measured by c50 was not affected by eye position. For dark bars, there was a statistically significant decline in the normalized Rmax with increasing deviation from straight ahead. Data for bright bars showed a similar trend with a less rapid decline. Our results indicate that neurons representing the central retina show a bias for the straight-ahead position resulting from modulation of the response gain without an accompanying modulation of contrast sensitivity. The modulation is especially obvious for dark stimuli, which might be useful for directing attention to hazardous situations such as dark holes or shadows concealing important objects
Selective activation of visual cortex neurons by fixational eye movements: Implications for neural coding
No full textDuring normal vision, when subjects attempt to fix their gaze on a small stimulus feature, small fixational eye movements persist. We have recorded the impulse activity of single neurons in primary visual cortex (V1) of macaque monkeys while their fixational eye movements moved the receptive-field activating region (AR) over and around a stationary stimulus. Three types of eye movement activation were found. (1) Saccade cells discharged when a fixational saccade moved the AR onto the stimulus, off the stimulus, or across the stimulus. (2) Position/drift cells discharged during the intersaccadic (drift) intervals and were not activated by saccades that swept the AR across the stimulus without remaining on it. To activate these neurons, it was essential that the AR be placed on the stimulus and many of these cells were selective for the sign of contrast. They had smaller ARs than the other cell types. (3) Mixed cells fired bursts of activity immediately following saccades and continued to fire at a lower rate during intersaccadic intervals. The tendency of each neuron to fire transient bursts or sustained trains of impulses following saccades was strongly correlated with the transiency of its response to stationary flashed stimuli. For one monkey, an extraretinal influence accompanying fixational saccades was identified. During natural viewing, the different eye movement classes probably make different contributions to visual processing. Position/drift neurons are well suited for coding spatial details of the visual scene because of their small AR size and their selectivity for sign of contrast and retinal position. However, saccade neurons transmit information that is ambiguous with respect to the spatial details of the retinal image because they are activated whether the AR lands on a stimulus contour, or the AR leaves or crosses the contour and lands in another location. Saccade neurons may be involved in constructing a stable world in spite of incessant retinal image motion, as well as in suppressing potentially confusing input associated with saccades
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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