In 1958, David Hubel and Torsten Wiesel discovered that cells in the visual cortex of cats respond to bars of light moving in a specific direction. Their discovery of these cells, termed as ‘orientation selective cells’, has revolutionized the whole field of neuroscience.
This recent paper peers into the origin of diversity of orientation selectivity in the visual cortex.
- ‘Simple cells’ of visual cortex (V1) respond to the orientation of edges.
- This response comes from the converging synaptic inputs of thalamic neurons, which in turn receive inputs from retinal cells.
- Spike thresholds of the V1 cells determine the sharpness of the orientation tuning curve.
- V1 cells have been shown to be interconnected, with a bias towards cells that respond to similar orientations.
- Orientation selective cells are arranged in columns giving rise to an ‘orientation map’ on the surface of the visual cortex.
- This map resembles multiple pinwheels.
- The cells in the center of the pinwheels have broader orientation curves owing to the inputs they receive from neighbouring cells.
A range of orientation selective cells has been observed in the visual cortex. This paper investigates the source of this variability.
Wilson et al., delineate three hypothetical sources of the diversity: 1) differences in synaptic input , 2) differences in spike threshold and 3) differences in dendritic integration.
The authors express the calcium sensor, GCamP6 in the visual cortex of ferrets and perform 1) in vivo 2-photon imaging and 2) in vivo whole cell recordings to test their hypotheses.
They have reported that this diversity stems from neither the differences in synaptic input nor the spike threshold, but rather the functional clustering of synapses on dendrites.
In-vivo 2-photon imaging of dendritic spines and the soma of V1 cells while presenting a stimulus of bars moving in different directions revealed that-:
- orientation selectivity does not correlate with the location of cells in the orientation map. (Orientation map arises from orientation columns. And, these cells differ because they receive inputs from different sets of thalamic neurons)
- it correlates with the level of non-linearity of the cell’s input-output function.
An important source of non-linearity of V1 cells is the spike threshold, which determines the sharpness of the orientation curve. The authors performed in-vivo whole cell recordings and found that-:
- orientation selectivity does not correlate with spike threshold.
Wilson et al., then go on to show that functional clustering of synaptic inputs to the dendrites of V1 cells correlates with orientation selectivity.
Clustering of synaptic inputs provides local amplification of inputs, which shapes the orientation selectivity.