Showing papers by "David J. Heeger published in 2007"

Brain areas selective for both observed and executed movements.

Abstract: When observing a particular movement a subset of movement-selective visual and visuomotor neurons are active in the observer's brain, forming a representation of the observed movement. Similarly, w...

Neural Correlates of Sustained Spatial Attention in Human Early Visual Cortex

Abstract: Attention is thought to enhance perceptual performance at attended locations through top-down attention signals that modulate activity in visual cortex. Here, we show that activity in early visual cortex is sustained during maintenance of attention in the absence of visual stimulation. We used functional magnetic resonance imaging (fMRI) to measure activity in visual cortex while human subjects performed a visual detection task in which a variable-duration delay period preceded target presentation. Portions of cortical areas V1, V2, and V3 representing the attended part of the visual field exhibited sustained increases in activity throughout the delay period. Portions of these cortical areas representing peripheral, unattended parts of the visual field displayed sustained decreases in activity. The data were well fit by a model that assumed the sustained neural activity was constant in amplitude over a time period equal to that of the actual delay period for each trial. These results demonstrate that sustained attention responses are present in early visual cortex (including primary visual cortex), in the absence of a visual stimulus, and that these responses correlate with the allocation of visuospatial attention in both the spatial and temporal domains.

Hierarchy of cortical responses underlying binocular rivalry.

Abstract: During binocular rivalry, physical stimulation is dissociated from conscious visual awareness. Human brain imaging reveals a tight linkage between the neural events in human primary visual cortex (V1) and the dynamics of perceptual waves during transitions in dominance during binocular rivalry. Here, we report results from experiments in which observers' attention was diverted from the rival stimuli, implying that: competition between two rival stimuli involves neural circuits in V1, and attention is crucial for the consequences of this neural competition to advance to higher visual areas and promote perceptual waves.

Specificity of Human Cortical Areas for Reaches and Saccades

Abstract: Electrophysiological studies in monkeys have identified effector-related regions in the posterior parietal cortex (PPC). The lateral intraparietal area, for example, responds preferentially for saccades, whereas the parietal reach region responds preferentially for arm movements. However, the degree of effector selectivity actually observed is limited; each area contains neurons selective for the nonpreferred effector, and many neurons in both areas respond for both effectors. We used functional magnetic resonance imaging to assess the degree of effector preference at the population level, focusing on topographically organized regions in the human PPC [visual area V7, intraparietal sulcus 1 (IPS1), and IPS2]. An event-related design adapted from monkey experiments was used. In each trial, an effector cue preceded the appearance of a spatial target, after which a Go signal instructed subjects to produce the specified movement with the specified effector. Our results show that the degree of effector specificity is limited in many cortical areas and transitions gradually from saccade to reach preference as one moves through the hierarchy of areas in the occipital, parietal, and frontal cortices. Saccade preference was observed in visual cortex, including early areas and V7. IPS1 exhibited balanced activation to saccades and reaches, whereas IPS2 showed a weak but significant preference for reaches. In frontal cortex, areas near the central sulcus showed a clear and absolute preference for reaches, whereas the frontal eye field showed little or no effector selectivity. Although these results contradict many theoretical conclusions about effector specificity, they are compatible with the complex picture arising from electrophysiological studies and also with previous imaging studies that reported mostly overlapping saccade- and arm-related activation. The results are also compatible with theories of efficient coding in cortex.

Orientation-selective adaptation to illusory contours in human visual cortex

Abstract: Humans can perceive illusory or subjective contours in the absence of any real physical boundaries. We used an adaptation protocol to look for orientation-selective neural responses to illusory contours defined by phase-shifted abutting line gratings in the human visual cortex. We measured functional magnetic resonance imaging (fMRI) responses to illusory-contour test stimuli after adapting to an illusory-contour adapter stimulus that was oriented parallel or orthogonal to the test stimulus. We found orientation-selective adaptation to illusory contours in early (V1 and V2) and higher-tier visual areas (V3, hV4, VO1, V3A/B, V7, LO1, and LO2). That is, fMRI responses were smaller for test stimuli parallel to the adapter than for test stimuli orthogonal to the adapter. In two control experiments using spatially jittered and phase-randomized stimuli, we demonstrated that this adaptation was not just in response to differences in the distribution of spectral power in the stimuli. Orientation-selective adaptation to illusory contours increased from early to higher-tier visual areas. Thus, both early and higher-tier visual areas contain neurons selective for the orientation of this type of illusory contour.

Rapid and precise retinotopic mapping of the visual cortex obtained by voltage-sensitive dye imaging in the behaving monkey

Abstract: Retinotopy is a fundamental organizing principle of the visual cortex. Over the years, a variety of techniques have been used to examine it. None of these techniques, however, provides a way to rap...

Second-Order Patterns in Human Visual Cortex Orientation-Selective Adaptation to First- and

Abstract: [PDF] [Full Text] , February 1, 2006; 95 (2): 591-592. J Neurophysiol Z. Kourtzi Adaptation to Firstand Second-Order Patterns in Human Visual Cortex" Textures of Natural Images in the Human Brain. Focus on "Orientation-Selective [PDF] [Full Text] [Abstract] , December 20, 2006; 26 (51): 13128-13142. J. Neurosci. J. Larsson and D. J. Heeger Two Retinotopic Visual Areas in Human Lateral Occipital Cortex [PDF] [Full Text] [Abstract] , February 1, 2007; 97 (2): 1319-1325. J Neurophysiol H. Ashida, A. Lingnau, M. B. Wall and A. T. Smith Motion fMRI Adaptation Reveals Separate Mechanisms for First-Order and Second-Order [PDF] [Full Text] [Abstract] , February 28, 2007; 27 (9): 2186-2195. J. Neurosci. L. Montaser-Kouhsari, M. S. Landy, D. J. Heeger and J. Larsson Orientation-Selective Adaptation to Illusory Contours in Human Visual Cortex