Enhanced Category Tuning Revealed by Intracranial Electroencephalograms in High-Order Human Visual Areas

Eran Privman, Yuval Nir, Uri Kramer, Svetlana Kipervasser, Fani Andelman, Miri Y. Neufeld, Roy Mukamel, Yehezkel Yeshurun, Itzhak Fried, and Rafael Malach
The Journal of Neuroscience, June 6, 2007, 27(23):6234-6242;

The functional organization of human sensory cortex was studied by comparing intracranial EEG (iEEG) recordings of local field potentials in neurosurgical patients with functional magnetic resonance imaging (fMRI) obtained in healthy subjects. Using naturalistic movie stimuli, we found a tight correlation between these two measures throughout the human sensory cortex. Importantly, the correlation between the iEEG and fMRI signals was site-specific, exhibiting neuroanatomically specific coupling. In several cortical sites the iEEG activity was confined strictly to one object category. This site selectivity was not limited to faces but included other object categories such as houses and tools. The selectivity of the iEEG signals to images of different object categories was remarkably higher when compared with the selectivity of the corresponding fMRI signals. A plausible interpretation of the fMRI and iEEG results concerns cortical organization in which object categories are organized in a mosaic of narrowly tuned object-selective clusters.

Duration-Dependent fMRI Adaptation and Distributed Viewer-Centered Face Representation in Human Visual Cortex

Fang Fang, Scott O. Murray, Sheng He
Cerebral Cortex June 2007;17:1402--1411 doi:10.1093/cercor/bhl053

Two functional magnetic resonance imaging (fMRI) face viewpoint
adaptation experiments were conducted to investigate whether
fMRI adaptation in high-level visual cortex depends on the duration
of adaptation and how different views of a face are represented in
the human visual system. We found adaptation effects in multiple
face-selective areas, which suggest a distributed, viewer-centered
representation of faces in the human visual system. However, the
nature of the adaptation effects was dependent on the length of adaptation.
With long adaptation durations, face-selective areas along
the hierarchy of the visual system gradually exhibited viewpointtuned
adaptation. As the angular difference between the adapter
and test stimulus increased, the blood oxygen level--dependent
(BOLD) signal evoked by the test stimulus gradually increased as
a function of the amount of 3-dimensional (3D) rotation. With short
adaptation durations, however, face-selective areas in the ventral
pathway, including the lateral occipital cortex and right fusiform
area, exhibited viewpoint-sensitive adaptation. These areas showed
an increase in the BOLD signal with a 3D rotation, but this signal
increase was independent of the amount of rotation. Further, the
right superior temporal sulcus showed little or very weak viewpoint
adaptation with short adaptation durations. Our findings suggest
that long- and short-term fMRI adaptations may reflect selective
properties of different neuronal mechanisms.

The Cortical Representation of Objects Rotating in Depth

Sarah Weigelt, Zoe Kourtzi, Axel Kohler, Wolf Singer, Lars Muckli
The Journal of Neuroscience, April 4, 2007, 27(14):3864-3874;

The perception of motion provides valuable interpolations of the visual scene. This fundamental capacity of the visual system is evident in apparent rotation: by presenting only two images of an object rotated in space, a vivid illusion of a smooth apparent motion in three dimensions can be induced. The unseen interpolated rotation views are filled in by the visual system. In the present study, we identified the cortical network responsible for this filling-in process. We argue that cross talk between areas of the ventral and dorsal visual pathways promote the illusion of smooth apparent rotation. Most interestingly, the network represents the unseen object views. Using functional magnetic resonance adaptation, we are able to show that the cortical network selectively adapts to the illusory object views. Our findings provide strong evidence for cortical representations of three-dimensional rotating objects that are view invariant with respect to the rotation path. Furthermore, our results confirm psychophysical investigations that unseen interpolated rotation views can be primed by apparent motion. By applying functional magnetic resonance adaptation, we show for the first time cortical adaptation to unseen objects. Together, our neuroimaging study advances the understanding of the cortical mechanisms mediating the influence of motion on object processing.

In the Eye of the Beholder: Visual Experience and Categories in the Human Brain

Johannes Haushofer, Nancy Kanwisher
Neuron 53, March 15, 2007

How does experience change representations of visual objects in the brain? Do cortical object representations reflect category membership? In this issue of Neuron, Jiang et al. show that category training leads to sharpening of neural responses in high-level visual cortex; in contrast, category boundaries may be represented only in prefrontal cortex.

Categorization Training Results in Shapeand Category-Selective Human Neural Plasticity

Xiong Jiang, Evan Bradley, Regina A. Rini, Thomas Zeffiro, John VanMeter, Maximilian Riesenhuber
Neuron 53, 891–903, March 15, 2007

Object category learning is a fundamental ability, requiring the combination of ‘‘bottom-up’’ stimulus-driven with ‘‘top-down’’ task-specific information. It thereforemay be a fruitful domain for study of the general neural mechanisms
underlying cortical plasticity. A simple model predicts that category learning involves the formation of a task-independent shape-selective representation that provides input to circuits learning the categorization task, with the computationally
appealing prediction of facilitated learning of additional, novel tasks over the
same stimuli. Using fMRI rapid-adaptation techniques, we find that categorization training (on morphed ‘‘cars’’) induced a significant release from adaptation for small shape changes in lateral occipital cortex irrespective of category membership, compatible with the sharpening of a representation coding for physical appearance.
In contrast, an area in lateral prefrontal cortex, selectively activated during ategorization, showed sensitivity posttraining to explicit changes in category membership. Further supporting the model, categorization training also improved discrimination performance on the trained stimuli.

Orientation-Selective Adaptation to Illusory Contours in Human Visual Cortex

Leila Montaser-Kouhsari, Michael S. Landy, David J. Heeger, Jonas Larsson
The Journal of Neuroscience, February 28, 2007, 27(9):2186-2195; doi:10.1523/JNEUROSCI.4173-06.2007

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.

Fulltext: http://www.jneurosci.org/cgi/reprint/27/9/2186

Functional imaging reveals visual modulation of specific fields in auditory cortex

Kayser C, Petkov CI, Augath M, Logothetis NK
J Neurosci. 2007 Feb 21;27(8):1824-35

Merging the information from different senses is essential for successful interaction with real-life situations. Indeed, sensory integration can reduce perceptual ambiguity, speed reactions, or change the qualitative sensory experience. It is widely held that integration occurs at later processing stages and mostly in higher association cortices; however, recent studies suggest that sensory convergence can occur in primary sensory cortex. A good model for early convergence proved to be the auditory cortex, which can be modulated by visual and tactile stimulation; however, given the large number and small size of auditory fields, neither human imaging nor microelectrode recordings have systematically identified which fields are susceptible to multisensory influences. To reconcile findings from human imaging with anatomical knowledge from nonhuman primates, we exploited high-resolution imaging (functional magnetic resonance imaging) of the macaque monkey to study the modulation of auditory processing by visual stimulation. Using a functional parcellation of auditory cortex, we localized modulations to individual fields. Our results demonstrate that both primary (core) and nonprimary (belt) auditory fields can be activated by the mere presentation of visual scenes. Audiovisual convergence was restricted to caudal fields [prominently the core field (primary auditory cortex) and belt fields (caudomedial field, caudolateral field, and mediomedial field)] and continued in the auditory parabelt and the superior temporal sulcus. The same fields exhibited enhancement of auditory activation by visual stimulation and showed stronger enhancement for less effective stimuli, two characteristics of sensory integration. Together, these findings reveal multisensory modulation of auditory processing prominently in caudal fields but also at the lowest stages of auditory cortical processing.

PMID: 17314280

Spatio-temporal point-spread function of fMRI signal in human gray matter at 7 Tesla

Shmuel A, Yacoub E, Chaimow D, Logothetis NK, Ugurbil K.
Neuroimage. 2007 Jan 4;

This study investigated the spatio-temporal properties of blood-oxygenation level-dependent (BOLD) functional MRI (fMRI) signals in gray matter, excluding the confounding, inaccurate contributions of large blood vessels. Specifically, we quantified the spatial specificity of the BOLD response, and we investigated whether this specificity varies as a function of time from stimulus onset. fMRI was performed at 7 Tesla (T), where mapping signals of parenchymal origin are easily detected. Two abutting visual stimuli were adjusted to elicit responses centered on a flat gray matter region in V1. fMRI signals were sampled at high-resolution orthogonal to the retinotopic boundary between the representations of the stimuli. Signals from macro-vessels were masked out. Principal component analysis revealed that the first component in space accounted for 96.2+/-1.6% of the variance over time. The spatial profile of this time-invariant response was fitted with a model consisting of the convolution of a step function and a Gaussian point-spread-function (PSF). The mean full-width at half-maximal-height of the fitted PSF was 2.34+/-0.20 mm. Based on simulations of confounding effects, we estimate that BOLD PSF in human gray matter is smaller than 2 mm. A detailed time-point to time-point analysis revealed that the estimated PSF obtained during the 3rd (1.52 mm) and 4th (1.99 mm) seconds of stimulation were narrower than the mean estimated PSF obtained from the 5th second on (2.42+/-0.15 mm, mean+/-SD). The position of the edge of the responding region was offset (1.72+/-0.07 mm) from the boundary of the stimulated region, indicating a spatial non-linearity. Simulations showed that the effective contrast between active and non-active columns is reduced 25-fold when imaged using a PSF whose width is equal to the cycle of the imaged columnar organization. Thus, the PSF of the hyper-oxygenated BOLD response in human gray matter is narrower than that reported at 1.5 T, where macro-vessels dominate the mapping signals. The initial phase of this response is more spatially specific than later phases. Data acquisition methods that suppress macro-vascular signals should increase the spatial specificity of BOLD fMRI. The choice of optimal stimulus duration represents a trade-off between the spatial specificity and the overhead associated with short stimulus duration.

PMID: 17306989
Free fulltext: Science Direct

Neural coding of reward prediction error signals during classical conditioning with attractive faces

Bray SL, O'doherty JP.
J Neurophysiol. 2007 Feb 15;
PMID: 17303809

Attractive faces can be considered to be a form of visual reward. Previous imaging studies have reported activity in reward structures including orbitofrontal cortex and nucleus accumbens during presentation of attractive faces. Given that these stimuli appear to act as rewards, we set out to explore whether it was possible to establish conditioning in human subjects by pairing presentation of arbitrary affectively neutral stimuli with subsequent presentation of attractive and unattractive faces. Furthermore, we scanned human subjects with fMRI while they underwent this conditioning procedure in order to determine whether a reward prediction error signal is engaged during learning with attractive faces, as is known to be the case for learning with other types of reward such as juice and money. Subjects showed conditioning-related changes in behavioral ratings to the CS stimuli, notably for those CSs paired with attractive female faces. We used a Rescorla-Wagner learning rule to generate a reward prediction error signal, entered as a regressor in our fMRI analysis. We found significant prediction error-related activity in the ventral striatum during conditioning with attractive compared to unattractive faces. These findings suggest that an arbitrary stimulus can acquire conditioned value by being paired with pleasant visual stimuli just as with other types of reward such as money or juice. The findings we describe here may provide insights into the neural mechanisms tapped into by advertisers seeking to influence behavioral preferences by repeatedly exposing consumers to simple associations between products and rewarding visual stimuli such as pretty faces.

Distinct and Convergent Visual Processing of High and Low Spatial Frequency Information in Faces

Rotshtein P, Vuilleumier P, Winston J, Driver J, Dolan R.
Cereb Cortex. 2007 Feb 5

We tested for differential brain response to distinct spatial frequency (SF) components in faces. During a functional magnetic resonance imaging experiment, participants were presented with "hybrid" faces containing superimposed low and high SF information from different identities. We used a repetition paradigm where faces at either SF range were independently repeated or changed across consecutive trials. In addition, we manipulated which SF band was attended. Our results suggest that repetition and attention affected partly overlapping occipitotemporal regions but did not interact. Changes of high SF faces increased responses of the right inferior occipital gyrus (IOG) and left inferior temporal gyrus (ITG), with the latter response being also modulated additively by attention. In contrast, the bilateral middle occipital gyrus (MOG) responded to repetition and attention manipulations of low SF. A common effect of high and low SF repetition was observed in the right fusiform gyrus (FFG). Follow-up connectivity analyses suggested direct influence of the MOG (low SF), IOG, and ITG (high SF) on the FFG responses. Our results reveal that different regions within occipitotemporal cortex extract distinct visual cues at different SF ranges in faces and that the outputs from these separate processes project forward to the right FFG, where the different visual cues may converge.

PMID: 17283203

Free Fulltext: http://cercor.oxfordjournals.org/cgi/reprint/bhl180v1

The Effect of Spatial Attention on Contrast Response Functions in Human Visual Cortex

Giedrius T. Buracas and Geoffrey M. Boynton

Previous electrophysiology data suggests that the modulation of neuronal firing by spatial attention depends on stimulus contrast, which
has been described using either a multiplicative gain or a contrast-gain model. Herewemeasured the effect of spatial attention on contrast
responses in humans using functional MRI. To our surprise, we found that the modulation of blood oxygenation level-dependent (BOLD)
responses by spatial attention does not greatly depend on stimulus contrast in visual cortical areas tested [V1, V2, V3, andMT
(middle
temporal area)]. An additive model, rather than a multiplicative or contrast-gain model best describes the attentional modulations in V1.
This inconsistency with previous single-unit electrophysiological data has implications for the population-based neuronal source of the
BOLD signal.