Dr. Eric S. Fortune, PhD
  Department of Biological Sciences
  New Jersey Institute of Technology
  Newark, NJ 07102-1982 USA
  Tel: +1 443 312 9610, FAX: +1 973 596 5689
  e-mail: eric.fortune@njit.edu


Research program: electric fish
Research program: songbirds


Direction Selectivity

A fundamental computation in motion processing is the identification of the direction of motion of moving sensory images. So called 'direction selective' neurons are interesting neural correlates in this regard - direction selective neurons respond preferentially to one direction of image motion over others. David Hubel and Torsten Wiesel won the Nobel Prize in part for their work describing direction selective neurons in cat visual cortex. Since this discovery, direction selective responses have been found in a wide array of sensory modalities across taxa.

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Such direction selective neurons may be important in the control of locomotion in a variety of contexts, including refuge tracking and prey capture. Further, we have found that direction selectivity can be modulated via a unique category of sensory stimuli that result from social behavior in these fish. The mechanisms for direction selectivity, however, are less well understood. Theoretically, the generation of direction selectivity requires two features (commonly known as a Reichardt detector). The first is convergence of information from a minimum of two spatially distinct regions on a receptor array with different time constants or delays. The second is a non-linear integrator. This combination of mechanisms has not, to our knowledge, been described in actual nervous systems of any animal species.

Over the past few years, my colleagues (see below) and I have tackled this problem in weakly electric fish, and have shown that a combination of differences in short-term synaptic depression and T-type calcium conductances are used in a Reichardt circuit. More recently we have been studying how these mechanisms might give rise to coding strategies for foreground and background images.


Relevant publications

Khosravi-Hashemi, N., Fortune, E.S., and M.J. Chacron (2011) Coding movement direction by burst firing in electrosensory neurons. PMID:21775723.

Direction selective responses are carried primarily by bursts of action potentials. As a result, if all of the spikes are used to measure direction selective responses, the level of directionality can be significantly less than that coded by the bursts alone. In this paper we include a model that uses an interaction between the receptive field organization and T-type Calcium channels to generate selective burst activity. We also include a model that selectively extracts burst activity in downstream decoders.
Chacron, M.J. and E.S. Fortune (2010) Subthreshold membrane conductances enhance directional selectivity in vertebrate sensory neurons. J. Neurophysiol., 4(2):e32, PMID:20445028.

Reichardt motion detectors require two features for direction selectivity: different delays of information from different spatial locations on the sensor array and a non-linear integrator of this information. This manuscript shows that the non-linear integrator for direction selectivity in midbrain electrosensory neurons is mediated by a T-type calcium channel.
Chacron, M.J., Toporikova, N., and E.S. Fortune (2009) Differences in the time course of short-term depression across receptive fields are correlated with directional selectivity in electrosensory neurons., J. Neurophysiol., 102:3270-3279, PMID:19793877.

Reichardt motion detectors require two features for direction selectivity: different delays of information from different spatial locations on the sensor array and a non-linear integrator of this information. This manuscript shows that the differences in the delays from different locations on the receptor array are mediated in midbrain electrosensory neurons is mediated by differences in the time course of short-term synaptic depression.
Carver, S., Roth, E., Cowan, N.J. and E.S. Fortune (2008) Synaptic plasticity can produce and enhance direction selectivity. PLoS Comput. Biol., 4(2):e32, PMID:18282087.

A model that demonstrates how two time constants associated with short-term synaptic depression - a fast time constant on the order of milliseconds to seconds and a slower time constant on the order of seconds to tens of seconds - contribute to the generation and enhancement of direction selectivity. The model captures the major features observed in a population of direction-selective midbrain neurons in the electrosensory midbrain of Eigenmannia virescens.
Ramcharitar, J.U., Tan, E.W., E.S. Fortune (2006) Global electrosensory oscillations enhance directional responses of midbrain neurons in Eigenmannia. J. Neurophys., PMID:16790600.

Characterizes the responses of midbrain neurons to moving objects in the presence and absence of post-Jamming-Avoidance-Response global stimuli. Remarkably, gamma band interference seems to enhance direction selectivity in these neurons. This phenomenon is strongly correlated with a measure of short-term synaptic depression in these neurons.
Ramcharitar, J.U., Tan, E.W., and E.S. Fortune (2005) Effects of global electrosensory signals on motion processing in the midbrain of Eigenmannia. J. Comp. Physiol. A, 191:865-872, PMID:16001182.

Characterizes the magnitudes of the responses of midbrain neurons to moving objects in the presence and absence of global electrosensory stimuli.
@ericfortunephd 11-Dec-2015