Biology subjects

Rajan, R.

Publications and source records attributed to Rajan, R..

5 recordsLinked to original sources

bioRxiv2018

Unidirectional monosynaptic connections from auditory areas to the primary visual cortex in the marmoset monkey

Until the late 20th Century, it was believed that different sensory modalities were processed by largely independent pathways in the primate cortex, with cross-modal integration only occurring in specialized polysensory areas. This model was challenged by the finding that the peripheral representation of the primary visual cortex (V1) receives monosynaptic connections from areas of the auditory cortex in the macaque. However, auditory projections to V1 have not been reported in other primates. We investigated the existence of direct interconnections between V1 and auditory areas in the marmoset, a New World monkey. Labelled neurons in auditory cortex were observed following 4 out of 10 retrograde tracer injections involving V1. These projections to V1 originated in the caudal subdivisions of auditory cortex (primary auditory cortex, caudal belt and parabelt areas), and targeted parts of V1 that represent parafoveal and peripheral vision. Injections near the representation of the vertical meridian of the visual field labelled few or no cells in auditory cortex. We also placed 8 retrograde tracer injections involving core, belt and parabelt auditory areas, none of which revealed direct projections from V1. These results confirm the existence of a direct, nonreciprocal projection from auditory areas to V1 in a different primate species, which has evolved separately from the macaque for over 30 million years. The essential similarity of these observations between marmoset and macaque indicate that early-stage audiovisual integration is a shared characteristic of primate sensory processing.

neuroscience↗

bioRxiv2018

Distance-dependent changes in courtship song amplitude reflect song state changes

Many animals increase the amplitude of their vocalizations as distance from a receiver increases. However, behavioral mechanisms underlying this increase remain unclear. Here, we addressed this using head-attached microphones to accurately record song amplitude in male zebra finches while presenting a female at difference distances. We show that individual courtship song syllables either increased (5/13) or decreased their amplitude (4/13) as distance from the female increased. Both increases and decreases were part of more general acoustic changes that resulted in properties more similar to those of \"undirected\" songs typically produced in the absence of a female. Increasing distance also reduced female responses to songs and absence of female responses reduced courtship song amplitude and number of songs per bout. These data suggest a simple behavioral mechanism where distance-dependent song amplitude changes reflect song state changes. Such state changes may be a general mechanism underlying distance-dependent amplitude changes in other organisms.

neuroscience↗

bioRxiv2017

Auditory motion does not modulate spiking activity in visual motion processing areas MT and MST

The integration of multiple sensory modalities is one of the key aspects of brain function, allowing animals to take advantage of concurrent sources of information to make more accurate perceptual judgments. For many years, it was thought that multisensory integration in the cerebral cortex only occurs in high-level \"polysensory\" association areas, but recent studies have demonstrated cross-modal influences in regions that were traditionally designated as unimodal. In particular, several human neuroimaging studies have reported that extrastriate areas involved in visual motion perception are also activated by auditory motion, and may integrate audio-visual motion cues. However, the exact nature and extent of the effects of auditory motion on the visual cortex have not been studied at the single neuron level. We recorded the spiking activity of neurons in the middle temporal (MT) and medial superior temporal (MST) areas of anesthetized marmoset monkeys upon presentation of unimodal stimuli (moving auditory or visual patterns), as well as bimodal stimuli (concurrent audio-visual motion). Despite robust, direction selective responses to visual motion, none of the sampled neurons responded to auditory motion stimuli. Moreover, concurrent moving auditory stimuli had no significant effect on the ability of single MT and MST neurons, or populations of simultaneously recorded neurons, to discriminate the direction of motion of visual stimuli (moving random dot patterns with varying levels of motion noise). Our findings do not support the hypothesis that direct interactions between MT, MST and areas low in the hierarchy of auditory areas underlie audiovisual motion integration.\n\nSignificance StatementMany studies have demonstrated that brain regions originally thought to be unisensory may play a role in multisensory processing. For example, some neuroimaging studies have found activity in regions involved in the processing of visual motion can be modified by auditory motion. We tested whether the spiking activity of neurons in two visual motion processing areas of the primate brain, areas MT and MST, can be modulated by moving auditory stimuli. Our results revealed that neurons in these areas neither respond to auditory motion, nor change their responses to visual motion according to auditory motion along the frontoparallel plane. These findings call into question the idea that audio-visual integration occurs at early stages of processing in the extrastriate cortex.

neuroscience↗

bioRxiv2017

Sensitivity of Neurons in the Middle Temporal Area of Marmoset Monkeys to Random Dot Motion

Neurons in the Middle Temporal area (MT) of the primate cerebral cortex respond to moving visual stimuli. The sensitivity of MT neurons to motion signals can be characterized by using random-dot stimuli, in which the strength of the motion signal is manipulated by adding different levels of noise (elements that move in random directions). In macaques, this has allowed the calculation of \"neurometric\" thresholds. We characterized the responses of MT neurons in sufentanil/nitrous oxide anesthetized marmoset monkeys, a species which has attracted considerable recent interest as an animal model for vision research. We found that MT neurons show a wide range of neurometric thresholds, and that the responses of the most sensitive neurons could account for the behavioral performance of macaques and humans. We also investigated factors that contributed to the wide range of observed thresholds. The difference in firing rate between responses to motion in the preferred and null directions was the most effective predictor of neurometric threshold, whereas the direction tuning bandwidth had no correlation with the threshold. We also showed that it is possible to obtain reliable estimates of neurometric thresholds using stimuli that were not highly optimized for each neuron, as is often necessary when recording from large populations of neurons with different receptive field concurrently, as was the case in this study. These results demonstrate that marmoset MT shows an essential physiological similarity to macaque MT, and suggest that its neurons are capable of representing motion signals that allow for comparable motion-in-noise judgments.\n\nNew and NoteworthyWe report the activity of neurons in marmoset MT in response to random-dot motion stimuli of varying coherence. The information carried by individual MT neurons was comparable to that of the macaque, and that the maximum firing rates were a strong predictor of sensitivity. Our study provides key information regarding the neural basis of motion perception in the marmoset, a small primate species that is becoming increasingly popular as an experimental model.

neuroscience↗

bioRxiv2016

A mathematical approach to correlating objective spectro-temporal features of environmental sounds with their subjective perceptions.

Many studies on the subjective perception of non-linguistic sounds have focused on only a single percept, e.g. pleasantness. In the present study, we have examined three different perception-related factors to also allow us to look at any inter-relationships between them that could be related to objective features. Objective features of the NLSs in this database were calculated and subjective perceptions were recorded from participants. These two elements - objective features and subjective perceptions - were then mapped together using various statistical and mathematical techniques. So as to ground our results in a meaningful context, we chose to map a set of objective features to human percepts which had been used in previous studies of NLS perception, but which had not yet been related back to objective features or combinations thereof.

neuroscience↗