Thursday, August 28, 2008

Emergence of binocular functional properties in a monocular neural circuit

Pavan Ramdya, Florian Engert
Nature Neuroscience 11, 1083 - 1090 (2008)

Sensory circuits frequently integrate converging inputs while maintaining precise functional relationships between them. For example, in mammals with stereopsis, neurons at the first stages of binocular visual processing show a close alignment of receptive-field properties for each eye. Still, basic questions about the global wiring mechanisms that enable this functional alignment remain unanswered, including whether the addition of a second retinal input to an otherwise monocular neural circuit is sufficient for the emergence of these binocular properties. We addressed this question by inducing a de novo binocular retinal projection to the larval zebrafish optic tectum and examining recipient neuronal populations using in vivo two-photon calcium imaging. Notably, neurons in rewired tecta were predominantly binocular and showed matching direction selectivity for each eye. We found that a model based on local inhibitory circuitry that computes direction selectivity using the topographic structure of both retinal inputs can account for the emergence of this binocular feature.

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Direction of Visual Apparent Motion Driven Solely by Timing of a Static Sound

Elliot Freeman, and Jon Driver
Current Biology, Vol 18, 1262-1266, 26 August 2008

In temporal ventriloquism, auditory events can illusorily attract perceived timing of a visual onset [1, 2, 3]. We investigated whether timing of a static sound can also influence spatio-temporal processing of visual apparent motion, induced here by visual bars alternating between opposite hemifields. Perceived direction typically depends on the relative interval in timing between visual left-right and right-left flashes (e.g., rightwards motion dominating when left-to-right interflash intervals are shortest [4]). In our new multisensory condition, interflash intervals were equal, but auditory beeps could slightly lag the right flash, yet slightly lead the left flash, or vice versa. This auditory timing strongly influenced perceived visual motion direction, despite providing no spatial auditory motion signal whatsoever. Moreover, prolonged adaptation to such auditorily driven apparent motion produced a robust visual motion aftereffect in the opposite direction, when measured in subsequent silence. Control experiments argued against accounts in terms of possible auditory grouping, or possible attention capture. We suggest that the motion arises because the sounds change perceived visual timing, as we separately confirmed. Our results provide a new demonstration of multisensory influences on sensory-specific perception [5], with timing of a static sound influencing spatio-temporal processing of visual motion direction.

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Believing is seeing: expectations alter visual awareness

Philipp Sterzer, Chris Frith, and Predrag Petrovic
Current Biology, Vol 18, R697-R698, 26 August 2008

Expectations have been shown to be powerful modulators of pain [1] and emotion [2] in placebo studies. In such experiments, expectations are induced by instructions combined with manipulation of the sensory experience that is unknown to the subjects. After an expectation learning phase where a painful stimulation is surreptitiously lowered following placebo application, the placebo effectively reduces subjective pain intensity in a subsequent test phase [3]. The strength of this placebo effect is closely related to the induced expectation [4]. Here, we asked whether this powerful cognitive bias reflects a general property of sensory information processing and tested whether the contents of visual awareness could be altered by a placebo-like expectation manipulation. We found a dramatic effect of experimentally induced expectations on the perception of an ambiguous visual motion stimulus. This shows that expectations have a strong and general influence on our experience of the sensory input independently of its specific type and content.

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Neural basis for unique hues

Cleo M. Stoughton and Bevil R. Conway
Current Biology, 2008, 18:16:R700-R702

All colors can be described in terms of four non-reducible ‘unique’ hues: red, green, yellow, and blue [1]. These four hues are also the most common ‘focal’ colors — the best examples of color terms in language [2]. The significance of the unique hues has been recognized since at least the 14th century [3] and is universal [4, 5], although there is some individual variation [6, 7]. Psychophysical linking hypotheses predict an explicit neural representation of unique hues at some stage of the visual system, but no such representation has been described [8]. The special status of the unique hues “remains one of the central mysteries of color science” [9]. Here we report that a population of recently identified cells in posterior inferior temporal cortex of macaque monkey contains an explicit representation of unique hues.

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Saturday, August 9, 2008

The Orientation Selectivity of Color-Responsive Neurons in Macaque V1

Elizabeth N. Johnson, Michael J. Hawken, Robert Shapley
The Journal of Neuroscience, August 6, 2008, 28(32):8096-8106; doi:10.1523/JNEUROSCI.1404-08.2008

Form has a strong influence on color perception. We investigated the neural basis of the form–color link in macaque primary visual cortex (V1) by studying orientation selectivity of single V1 cells for pure color patterns. Neurons that responded to color were classified, based on cone inputs and spatial selectivity, into chromatically single-opponent and double-opponent groups. Single-opponent cells responded well to color but weakly to luminance contrast; they were not orientation selective for color patterns. Most double-opponent cells were orientation selective to pure color stimuli as well as to achromatic patterns. We also found non-opponent cells that responded weakly or not at all to pure color; most were orientation selective for luminance patterns. Double-opponent and non-opponent cells' orientation selectivities were not contrast invariant; selectivity usually increased with contrast. Double-opponent cells were approximately equally orientation selective for luminance and equiluminant color stimuli when stimuli were matched in average cone contrast. V1 double-opponent cells could be the neural basis of the influence of form on color perception. The combined activities of single- and double-opponent cells in V1 are needed for the full repertoire of color perception.

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Wednesday, August 6, 2008

Converging Neuronal Activity in Inferior Temporal Cortex during the Classification of Morphed Stimuli.

Akrami A, Liu Y, Treves A, Jagadeesh B.
Cereb Cortex. 2008 Jul 31.

How does the brain dynamically convert incoming sensory data into a representation useful for classification? Neurons in inferior temporal (IT) cortex are selective for complex visual stimuli, but their response dynamics during perceptual classification is not well understood. We studied IT dynamics in monkeys performing a classification task. The monkeys were shown visual stimuli that were morphed (interpolated) between pairs of familiar images. Their ability to classify the morphed images depended systematically on the degree of morph. IT neurons were selected that responded more strongly to one of the 2 familiar images (the effective image). The responses tended to peak approximately 120 ms following stimulus onset with an amplitude that depended almost linearly on the degree of morph. The responses then declined, but remained above baseline for several hundred ms. This sustained component remained linearly dependent on morph level for stimuli more similar to the ineffective image but progressively converged to a single response profile, independent of morph level, for stimuli more similar to the effective image. Thus, these neurons represented the dynamic conversion of graded sensory information into a task-relevant classification. Computational models suggest that these dynamics could be produced by attractor states and firing rate adaptation within the population of IT neurons.

PMID: 18669590

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fMRI and its interpretations: an illustration on directional selectivity in area V5/MT

Bartels A, Logothetis NK, Moutoussis K.
Trends Neurosci. 2008 Aug 2.

fMRI is a tool to study brain function noninvasively that can reliably identify sites of neural involvement for a given task. However, to what extent can fMRI signals be related to measures obtained in electrophysiology? Can the blood-oxygen-level-dependent signal be interpreted as spatially pooled spiking activity? Here we combine knowledge from neurovascular coupling, functional imaging and neurophysiology to discuss whether fMRI has succeeded in demonstrating one of the most established functional properties in the visual brain, namely directional selectivity in the motion-processing region V5/MT+. We also discuss differences of fMRI and electrophysiology in their sensitivity to distinct physiological processes. We conclude that fMRI constitutes a complement, not a poor-resolution substitute, to invasive techniques, and that it deserves interpretations that acknowledge its stand as a separate signal.

PMID: 18676033

Neural repetition suppression reflects fulfilled perceptual expectations.

Summerfield C, Trittschuh EH, Monti JM, Mesulam MM, Egner T.
Nat Neurosci. 2008 Aug 1.

Stimulus-evoked neural activity is attenuated on stimulus repetition (repetition suppression), a phenomenon that is attributed to largely automatic processes in sensory neurons. By manipulating the likelihood of stimulus repetition, we found that repetition suppression in the human brain was reduced when stimulus repetitions were improbable (and thus, unexpected). Our data suggest that repetition suppression reflects a relative reduction in top-down perceptual 'prediction error' when processing an expected, compared with an unexpected, stimulus.

PMID: 18677308

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Saturday, August 2, 2008

Multivariate patterns in object-selective cortex dissociate perceptual and physical shape similarity.

Haushofer J, Livingstone MS, Kanwisher N.
PLoS Biol. 2008 Jul 29;6(7):e187.

Prior research has identified the lateral occipital complex (LOC) as a critical cortical region for the representation of object shape in humans. However, little is known about the nature of the representations contained in the LOC and their relationship to the perceptual experience of shape. We used human functional MRI to measure the physical, behavioral, and neural similarity between pairs of novel shapes to ask whether the representations of shape contained in subregions of the LOC more closely reflect the physical stimuli themselves, or the perceptual experience of those stimuli. Perceptual similarity measures for each pair of shapes were obtained from a psychophysical same-different task; physical similarity measures were based on stimulus parameters; and neural similarity measures were obtained from multivoxel pattern analysis methods applied to anterior LOC (pFs) and posterior LOC (LO). We found that the pattern of pairwise shape similarities in LO most closely matched physical shape similarities, whereas shape similarities in pFs most closely matched perceptual shape similarities. Further, shape representations were similar across participants in LO but highly variable across participants in pFs. Together, these findings indicate that activation patterns in subregions of object-selective cortex encode objects according to a hierarchy, with stimulus-based representations in posterior regions and subjective and observer-specific representations in anterior regions.

PMID: 18666833

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Friday, August 1, 2008

Influence of Reward Delays on Responses of Dopamine Neurons

Shunsuke Kobayashi and Wolfram Schultz
The Journal of Neuroscience, July 30, 2008, 28(31):7837-7846; doi:10.1523/JNEUROSCI.1600-08.2008

Psychological and microeconomic studies have shown that outcome values are discounted by imposed delays. The effect, called temporal discounting, is demonstrated typically by choice preferences for sooner smaller rewards over later larger rewards. However, it is unclear whether temporal discounting occurs during the decision process when differently delayed reward outcomes are compared or during predictions of reward delays by pavlovian conditioned stimuli without choice. To address this issue, we investigated the temporal discounting behavior in a choice situation and studied the effects of reward delay on the value signals of dopamine neurons. The choice behavior confirmed hyperbolic discounting of reward value by delays on the order of seconds. Reward delay reduced the responses of dopamine neurons to pavlovian conditioned stimuli according to a hyperbolic decay function similar to that observed in choice behavior. Moreover, the stimulus responses increased with larger reward magnitudes, suggesting that both delay and magnitude constituted viable components of dopamine value signals. In contrast, dopamine responses to the reward itself increased with longer delays, possibly reflecting temporal uncertainty and partial learning. These dopamine reward value signals might serve as useful inputs for brain mechanisms involved in economic choices between delayed rewards.

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Complementary Contributions of Prefrontal Neuron Classes in Abstract Numerical Categorization

Ilka Diester and Andreas Nieder
The Journal of Neuroscience, July 30, 2008, 28(31):7737-7747; doi:10.1523/JNEUROSCI.1347-08.2008

The primate prefrontal cortex (PFC) plays a cardinal role in forming abstract categories and concepts. However, it remains elusive how this is accomplished and to what extent the interaction of functionally distinct neuron classes underlies this representation. Here, we inferred the major cortical cell types, putative pyramidal cells, and interneurons by characterizing the waveforms of action potentials recorded in monkeys performing a cognitively demanding numerosity categorization task. Putative interneurons responded much faster than cells classified as pyramidal neurons and exhibited a higher reliability of category discrimination, whereas putative pyramidal cells showed a higher degree of category selectivity. An analysis of the numerosity tuning profiles and the temporal interactions of adjacent neurons indicated that inhibitory input by putative interneurons shapes the tuning to numerical categories of putative PFC pyramidal cells. These findings favor feedforward mechanisms subserving cognitive categorization and help to clarify cellular interactions in PFC microcircuits.

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Task difficulty modulates the activity of specific neuronal populations in primary visual cortex

Yao Chen, Susana Martinez-Conde, Stephen L Macknik, Yulia Bereshpolova, Harvey A Swadlow, Jose-Manuel Alonso

Nature Neuroscience 11, 974 - 982 (2008)  | doi:10.1038/nn.2147

Spatial attention enhances our ability to detect stimuli at restricted regions of the visual field. This enhancement is thought to depend on the difficulty of the task being performed, but the underlying neuronal mechanisms for this dependency remain largely unknown. We found that task difficulty modulates neuronal firing rate at the earliest stages of cortical visual processing (area V1) in monkey (Macaca mulatta). These modulations were spatially specific: increasing task difficulty enhanced V1 neuronal firing rate at the focus of attention and suppressed it in regions surrounding the focus. Moreover, we found that response enhancement and suppression are mediated by distinct populations of neurons that differ in direction selectivity, spike width, interspike-interval distribution and contrast sensitivity. Our results provide strong support for center-surround models of spatial attention and suggest that task difficulty modulates the activity of specific populations of neurons in the primary visual cortex.

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The temporal precision of reward prediction in dopamine neurons.

Christopher D Fiorillo, William T Newsome, Wolfram Schultz
Nature Neuroscience 11, 966 - 973 (2008) Published online: 27 July 2008 doi:10.1038/nn.2159

Midbrain dopamine neurons are activated when reward is greater than predicted, and this error signal could teach target neurons both the value of reward and when it will occur. We used the dopamine error signal to measure how the expectation of reward was distributed over time. Animals were trained with fixed-duration intervals of 1–16 s between conditioned stimulus onset and reward. In contrast to the weak responses that have been observed after short intervals (1–2 s), activations to reward increased steeply and linearly with the logarithm of the interval. Results with varied stimulus-reward intervals suggest that the neural expectation was substantial after just half an interval had elapsed. Thus, the neural expectation of reward in these experiments was not highly precise and the precision declined sharply with interval duration. The neural precision of expectation appeared to be at least qualitatively similar to the precision of anticipatory licking behavior.

PMID: 18660807

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