Role of feedback in mammalian vision: a new hypothesis and a computational model.
(25/7911)
This paper presents a novel hypothesis on the function of massive feedback pathways in mammalian visual systems. We propose that the cortical feature detectors compete not for the right to represent the output at a point, but for exclusive rights to abstract and represent part of the underlying input. Feedback can do this very naturally. A computational model that implements the above idea for the problem of detection is presented and based on that we suggest a functional role for the thalamo-cortical loop during perception of lines. We show that the model successfully tackles the so called Cross problem. Based on some recent experimental results, we discuss the biological plausibility of our model. We also comment on the relevance of our hypothesis (on the role of feedback) to general sensory information processing and recognition. (+info)
Residual vision in the blind field of hemidecorticated humans predicted by a diffusion scatter model and selective spectral absorption of the human eye.
(26/7911)
The notion of blindsight was recently challenged by evidence that patients with occipital damage and contralateral field defects show residual islands of vision which may be associated with spared neural tissue. However, this possibility could not explain why patients who underwent the resection or disconnection of an entire cerebral hemisphere exhibit some forms of blindsight. We present here a model for the detection of intraocular scatter, which can account for human sensitivity values obtained in the blind field of hemidecorticated patients. The model demonstrates that, under controlled experimental conditions i.e. where the extraocular scatter is eliminated, Lambertian intraocular scatter alone can account for the visual sensitivities reported in these patients. The model also shows that it is possible to obtain a sensitivity in the blind field almost equivalent to that in the good field using the appropriate parameters. Finally, we show with in-vivo spectroreflectometry measurements made in the eyes of our hemidecorticated patients, that the relative drop in middle wavelength sensitivity generally obtained in the blind field of these patients can be explained by selective intraocular spectral absorption. (+info)
Development and organization of ocular dominance bands in primary visual cortex of the sable ferret.
(27/7911)
Thalamocortical afferents in the visual cortex of the adult sable ferret are segregated into eye-specific ocular dominance bands. The development of ocular dominance bands was studied by transneuronal labeling of the visual cortices of ferret kits between the ages of postnatal day 28 (P28) and P81 after intravitreous injections of either tritiated proline or wheat germ agglutinin-horseradish peroxidase. Laminar specificity was evident in the youngest animals studied and was similar to that in the adult by P50. In P28 and P30 ferret kits, no modulation reminiscent of ocular dominance bands was detectable in the pattern of labeling along layer IV. By P37 a slight fluctuation in the density of labeling in layer IV was evident in serial reconstructions. By P50, the amplitude of modulation had increased considerably but the pattern of ocular dominance bands did not yet appear mature. The pattern and degree of modulation of the ocular dominance bands resembled that in adult animals by P63. Flat mounts of cortex and serial reconstructions of layer IV revealed an unusual arrangement of inputs serving the two eyes in the region rostral to the periodic ocular dominance bands. In this region, inputs serving the contralateral eye were commonly fused along a mediolateral axis, rostral to which were large and sometimes fused patches of ipsilateral input. (+info)
The human amygdala plays an important role in gaze monitoring. A PET study.
(28/7911)
Social contact often initially depends on ascertaining the direction of the other person's gaze. We determined the brain areas involved in gaze monitoring by a functional neuroimaging study. Discrimination between the direction of gaze significantly activated a region in the left amygdala during eye-contact and no eye-contact tasks to the same extent. However, a region in the right amygdala was specifically activated only during the eye-contact task. Results confirm that the left amygdala plays a general role in the interpretation of eye gaze direction, and that the activity of the right amygdala of the subject increases when another individual's gaze is directed towards him. This suggests that the human amygdala plays a role in reading social signals from the face. (+info)
An empirical basis for Mach bands.
(29/7911)
Mach bands, the illusory brightness maxima and minima perceived at the initiation and termination of luminance gradients, respectively, are generally considered a direct perceptual manifestation of lateral inhibitory interactions among retinal or other lower order visual neurons. Here we examine an alternative explanation, namely that Mach bands arise as a consequence of real-world luminance gradients. In this first of two companion papers, we analyze the natural sources of luminance gradients, demonstrating that real-world gradients arising from curved surfaces are ordinarily adorned by photometric highlights and lowlights in the position of the illusory bands. The prevalence of such gradients provides an empirical basis for the generation of this perceptual phenomenon. (+info)
Mach bands as empirically derived associations.
(30/7911)
If Mach bands arise as an empirical consequence of real-world luminance profiles, several predictions follow. First, the appearance of Mach bands should accord with the appearance of naturally occurring highlights and lowlights. Second, altering the slope of an ambiguous luminance gradient so that it corresponds more closely to gradients that are typically adorned with luminance maxima and minima in the position of Mach bands should enhance the illusion. Third, altering a luminance gradient so that it corresponds more closely to gradients that normally lack luminance maxima and minima in the position of Mach bands should diminish the salience of the illusion. Fourth, the perception of Mach bands elicited by the same luminance gradient should be changed by contextual cues that indicate whether the gradient is more or less likely to signify a curved or a flat surface. Because each of these predictions is met, we conclude that Mach bands arise because the association elicited by the stimulus (the percept) incorporates these features as a result of past experience. (+info)
Modal behavior of cortical neural networks during visual processing.
(31/7911)
The network behavior of cortical cells during the processing of a light flash was characterized in an isolated, but functionally intact, turtle visual system. Rapid changes in intracellular membrane potential were monitored optically using a voltage-sensitive dye (VSD). Spatially coherent changes in membrane potential were determined by subjecting high-speed movies of the VSD signals to Karhunen-Loeve decomposition. In all experimental trials analyzed (n > 50), coherent activity was restricted to a small number of similar spatial patterns or modes. At least four modes (M(1,1), M(1,2), M(2,1), and M(2,2)) have an organizational structure similar to the normal modes of a vibrating membrane (drum). This empirical observation of modal activity provides a useful framework for analyzing the macroscopic behavior of cortical networks. (+info)
Dynamic representation of eye position in the parieto-occipital sulcus.
(32/7911)
Dynamic representation of eye position in the parieto-occipital sulcus. Area V6A, on the anterior bank of the parieto-occipital sulcus of the monkey brain, contains neurons sensitive both to visual stimulation and to the position and movement of the eyes. We examined the effects of eye position and eye movement on the activity of V6A neurons in monkeys trained to saccade to and fixate on target locations. Forty-eight percent of the neurons responded during these tasks. The responses were not caused by the visual stimulation of the fixation light because extinguishing the fixation light had no effect. Instead the neurons responded in relation to the position of the eye during fixation. Some neurons preferred a restricted range of eye positions, whereas others had more complex and distributed eye-position fields. None of these eye-related neurons responded before or during saccades. They all responded postsaccadically during fixation on the target location. However, the neurons did not simply encode the static position of the eyes. Instead most (88%) responded best after the eye saccaded into the eye-position field and responded significantly less well when the eye made a saccade that was entirely contained within the eye-position field. Furthermore, for many eye-position cells (45%), the response was greatest immediately after the eye reached the preferred position and was significantly reduced after 500 ms of fixation. Thus these neurons preferentially encoded the initial arrival of the eye into the eye-position field rather than the continued presence or the movement of the eye within the eye-position field. Area V6A therefore contains a representation of the position of the eye in the orbit, but this representation appears to be dynamic, emphasizing the arrival of the eye at a new position. (+info)