Sensory Integration in the Whisker Pathway

Whisker.jpg

This was the last major assignment I had to do for my bachelor’s degree and I received a HD! It is an annotated bibliography (11 papers) with a short summary:

Summary 

The reading topic to be covered in this assignment was “sensory integration in the whisker pathway”. This encompasses how information from the sensory system is translated into neural correlates that can be understood by the brain, and how the cortical representation of these sensory inputs can generate a behavioural response. The main model of choice is the rat whisker-barrel system because of its functional efficiency and anatomical characteristics, as the whisker region of the primary somatosensory cortex have clusters of neurons called “barrels”, each of which correspond primarily with one whisker [1]. The applicability to humans has been compared to the way people use their fingers to sense tactile information at times [2], although there are obviously many differences in terms of the function and sensorimotor feedback mechanisms used in fingertips that are not used in whiskers.

There are three main parts to sensory integration in the whisker pathway. First, it is important to know how the sensory input is translated into spikes, or neural coding. Secondly, it is important to understand how this is correlated to activity in higher cognitive areas, such as the somatosensory cortex. Finally, the correlation between neuronal activity and behavioural response to the stimulus is essential to know in order to understand the effects of sensory integration.

Rat whiskers (mystacial vibrissae) have many sensory receptors on them (in the follicle), innervated by hundreds of primary afferent fibres, and the signals generated by these have been correlated to anatomical and functional maps in the cortex [3]. The motion of the whiskers move the follicles, and the sensory receptors, which are the terminals of trigeminal cells, create action potential trains [4]. The neurons respond differently to different stimuli based on features such as speed of adaptation, location, direction, and velocity of whisker displacement, whisking phase, and contact or detachment [5]. The whiskers can be used to perceive space in three dimensions and identify the shape of objects, and can thereby categorise responses into ‘what’ and ‘where’ systems [6]. The whisker response can also be affected by the mode of sensing; rats can still their whiskers (receptive) or whisk them (generative) to get different information on their surroundings [5].

Once the mechanical information has been translated to spikes, they travel through the parallel pathways and the thalamus to the primary somatosensory cortex areas corresponding to the whisker, or the barrel cortex. It has been shown that a general linear model can be applied to the primary afferent response to sensory signals [7]. The features of whisker motion that the sensory system uses to construct sensations was found to be the product of both amplitude and frequency (Af), which means the speed of the whisker is required to perceive sensations [8]. Whisker movement seems to be initiated by the motor cortex (protraction) and the somatosensory cortex (retraction), which shows that sensory integration plays a role in generating movement independent of the motor system [9]. However, another paper suggests that the somatosensory information is relayed to the motor cortex, which correlates with whisker movement and so helps with ongoing sensory perception, especially in relation to fine adjustment of individual whisker movement [10]. The role of the motor cortex is called in to question in another paper, which states that although it has a role, it is not responsible for signalling the sensory cortex about the whisker position during whisking [11]. The same article does establish that spiking pattern can be generated by whisker motion. Most researchers call for more studies to be done, especially on active behaving rats instead of anaesthetised ones.

Annotated Bibliography

1. Adibi M, Arabzadeh E. A Comparison of Neuronal and Behavioral Detection and Discrimination Performances in Rat Whisker System; 2011:356-365.
Research Notes: The aim of this study was to understand the correlation between cortical neuron response and rat behaviour. Rats were anaesthetised so that neuronal activity could be recorded in the somatosensory cortex while vibrotactile stimuli was applied to the whiskers. The single neurons were measured for detection and discrimination. Rats were then trained to distinguish between two vibrations, and it was found that they were better at discriminating than detecting in both behavioural and neuronal response measurements. Faster whisking means higher firing rate. The brain activity was measured while the rats were anaesthetised, and perhaps that might change the data, and it is mentioned that there was no whisking in the anaesthetised condition. The paper is useful in understanding the connection between sensory experience, brain activity, and behaviour by measuring the neuronal and the behavioural responses. It shows that there are correlations between sensory integration and neuronal and behavioural responses.

2. Fassihi A, Akrami A, Esmaeili V, Diamond ME. Tactile perception and working memory in rats and humans. Proceedings of the National Academy of Sciences 2014;111:2331-2336.
Research Notes: Compares efficacy of working memory in rats (and their use of whiskers) and humans (and their use of fingertips) in sensory discrimination. Shows rodents have more capabilities than previously expected. Helpful in that it uses both human and rat test subjects and shows similarities between them.

3. Arabzadeh E, Heimendahl MV, Diamond M. Vibrissal texture decoding. Scholarpedia 2009;4.
Research Notes: This webpage aims to summarise the current knowledge on the correlates between whisking and texture sensation of surfaces using the vibrissae system in rats. Vibrissal texture decoding will help investigate the signals (and their interpretation) generated by sensory receptors in response to the environment. The data was collated through many different studies. Describes the long whiskers (focus of this paper) which have correlating anatomical and functional maps. Describes the thresholds of texture differentiation and duration required to make a decision. Concludes that the kinetic signature hypothesis for encoding texture is the most plausible one so far, but there are still missing pieces. The neuronal firing rate-to-whisker kinetics relation was outlined in anaesthetised mice, but not in alert ones. The paper outlines plausible read-out mechanisms (from spike train to discrimination behaviour) and further investigation in other areas such as sensorimotor integration. This chapter is useful as a clear summary of the research, and is written simply and without too much jargon or methodology.

4. Diamond ME, Arabzadeh E. Whisker sensory system – From receptor to decision. Progress in Neurobiology 2013;103:28-40.
Research Notes: This paper’s goal was to characterise the behaviour of the rats’ whiskers, their specific motions in response to the external stimulus, the translation of stimulus to neural coding in the sensory cortex, and what happens afterwards. It’s long-term goal was to study the pathway between stimulus and receptor to decision and behaviour. This was done using a number of different research methods, including prior research analysis and some extrapolation, electrical stimulation, the creation of a texture library which profiled whisker movements per texture, measurement of kinetic signature, recording of responses in first-order, recording cortical activity at the same time as whisker behaviour using a high speed cameras, and training rats in behaviour paradigms to discriminate between textures in both generative and receptive conditions. The article is useful as it gives a comprehensive outline of what has been found and what needs to be studied. It is mentioned that the whisker-barrel system is used as it is an expert sensory system with direct correlates between sensory input and behaviour, cortical stimulation, and resulting decision-making, and its anatomical and functional organisation is exquisite and well studied. It can therefore be used as good source material for future studies or to gain a better understanding of the research that has been done so far.

5. Maravall M, Diamond ME. Algorithms of whisker-mediated touch perception. Current Opinion in Neurobiology 2014;25:176-186.
Research Notes: This study aims to identify the computations used along the receptor-to-cortex pathway in converting physical signals to sensations. It summarises what is known about the follicles and how they respond to different stimuli, the role of the mechanoreceptors/trigeminal ganglion neurons, generative vs receptive sensing, differences in TG, VPM, and Barrel Cortex neuron firings, sparseness, response heterogeneity, spike timing, adaptation, barrel cortex population coordination, and how downstream neurons can use a simple decoding scheme using linear synaptic weighting to identify texture. The conclusion offers some experimental paradigms that could help solve some of the unanswered questions. This review is useful as it is an up-to-date collation of what is known about the first step in integration of sensory information; the translation of mechanical stimulus to cortical language. It’s a relatively short review as it covers a very specific area in detail, rather than what is known of the whole picture.

6. Diamond ME, von Heimendahl M, Knutsen PM, Kleinfeld D, Ahissar E. ‘Where’ and ‘what’ in the whisker sensorimotor system. Nat Rev Neurosci 2008;9:601-612.
Research Notes: This study summarises the information on the whisker sensory system and looks to present evidence on how external stimuli is represented in the brain, an in particular the “what” and “where” systems. Used behavioural studies on wake animals and recorded sensory receptor neurons in anaesthetised animals (artificially stimulated whiskers) and found that rats use their whiskers to perceive space in three dimensions, there may be three functional classes of primary sensory neurons (‘where’), and whiskers can identify the shape and texture of objects. The paper ended with an outline of two possibilities for the sensory system’s knowledge of the behaviour response, and then future research directions. The paper is useful in that it divides the different courses of information into two streams that can be looked at separately. Sensory integration in the whisker pathway is looked at in a broader sense, and more studies can be built upon the collated information.

7. Bale MR, Davies K, Freeman OJ, Ince RA, Petersen RS. Low-dimensional sensory feature representation by trigeminal primary afferents. The Journal of Neuroscience 2013;33:12003-12012.
Research Notes: The study aims to create a model to predict the primary afferent response to sensory signals. This is based on the premise that whisking and object contact with the whisker evoke spikes, that the pattern of action potentials travel from primary afferents to the cerebral cortex through parallel, trisynaptic pathways, and that primary afferents respond to whisker motion with reliable spike-timing precision. Creating a model helps clarify the relationship between whisker motion and afferent response. It was found that a generalised linear model could predict the responses of the afferents in both white noise and texture-induced whisker motion. The model was rigorously tested  for and it was found that it could predict both timing and amplitude of the peaks, although it was more accurate when predicting white noise. The authors note that an even more accurate model would take in to account slow time course features of the stimulus, and that the model should be tested on behaving animals. The aspects covered by the model were stimulus filtering, spike feedback, and whisker motion and velocity. Using this model and building upon it could be useful in future studies, as well as for understanding different sensory systems. The article was useful in that it outlined and created a model for the first step of sensory integration: translating mechanical stimuli to neuronal processing.

8. Adibi M, Diamond ME, Arabzadeh E. Behavioral study of whisker-mediated vibration sensation in rats. Proceedings of the National Academy of Sciences 2012;109:971-976.
Research Notes: This paper addressed the features of whisker motion that the sensory system uses to construct sensations by training rat models to discriminate between different sinusoidal vibratory models on the left and right whiskers. The focus was on keeping the frequency (f) or the amplitude (A) equal. It was found that vibrations were sensed as a product of Af, which means the speed of the whisker is used to perceive sensations. Both single and cortical ensembles of neurons encoded the Af using firing rates. The study also showed that the rats’ judgement can be wrong if the final Af of both sides are equal, even if the separate A and f were different, which can be one reason why rats are not always correct. The study was interesting in that it used behaving rats instead of anaesthetised rats, which gave different results to prior studies. Could not tell if the paper explicitly mentioned what sort of sensory perception was being used (receptive or generative) (probably generative). This article is useful in that it looks specifically at how rats use their whiskers to discriminate between vibrations, which is a function of the first step in understanding sensory integration. The authors end the paper by saying the firing rate, and not its individual components, is the most salient property.

9. Matyas F, Sreenivasan V, Marbach F, et al. Motor control by sensory cortex. Science 2010;330:1240-1243.
Research Notes: The authors aimed to study the role of the primary somatosensory barrel cortex in controlling movement, as opposed to motor cortex control. They mapped sensory activity using one whisker for both the motor and somatosensory cortex areas, then inactivated each area to record a change in retraction response. Results implied the motor cortex is only required for protraction, not retraction, and these two pathways used different routes in the brain stem to get to the whiskers. They posed the question of whether sensory cortex motor control is apparent in just the whisker system or whether it can be generalised. The article relates to the end product of sensory integration – behavioural response.

10. Ferezou I, Haiss F, Gentet LJ, Aronoff R, Weber B, Petersen CCH. Spatiotemporal Dynamics of Cortical Sensorimotor Integration in Behaving Mice. Neuron 2007;56:907-923.
Research Notes: The paper shows the involvement of the motor cortex in sensory perception through a feedback loop of sorts. This view seems to be different from views in other papers, which generally focus on other parts in the sensory integration of information gained through the whiskers. The study uses voltage-sensitive dye to image the interactions between somatosensory and motor cortexes. It concludes that information from a single whisker deflection goes to the somatosensory cortex, is then relayed to the motor cortex, which then correlates with whisker movement, and so helps in ongoing tactile sensory perception. The motor cortex may use sensory input to affect fine adjustment of individual whisker movements. Authors mentioned problems in imaging technique; change in membrane potential does not necessarily mean action potentials were produced, so sensory response may be less distributed than the study suggests. However, they believe this form of imaging is promising for future endeavours. The paper shows how sensory information may  be used as it is being integrated.

11. Fee MS, Mitra PP, Kleinfeld D. Central Versus Peripheral Determinants of Patterned Spike Activity in Rat Vibrissa Cortex During Whisking; 1997:1144-1149.
Research Notes: The researchers address the sources of input for the rat whisker-barrel cortex by recording single unit spike trains during whisking. They found that whisking occurs independently from sensory feedback and single neuron output may represent whisker position. The study concludes that it is not the motor cortex which plays the main role in signalling the sensory cortex about the position of the whisker during whisking, and that the spiking pattern comes from whisker motion. The article is useful in establishing a basis for the correlation between movement of the vibrissae and neuron activity in the primary somatosensory vibrissa cortex, and in giving some insight as to what the corollary discharge might be doing. Further research is required. The paper is a bit old and does not use terminology such as whisker-barrel system, etc.

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