Chapter 2 · Optional lab
The visual lab
Realistic illustrations of the eye and retina, an interactive model of receptive fields and clickable maps of the visual pathways. No scores or assessments – explore for as long as it helps.
Visual processing
Why can light beside a point change a cell's response to that point?
We follow the response of a single ganglion cell. Through lateral inhibition, activity in one part of a circuit can reduce the response in neighbouring pathways. A receptive field describes where in the visual field stimulation can change the selected cell's activity. It includes regions that increase the response and regions that decrease it.
ON and OFF are parallel biological pathways with opposite contrast preferences. Here we compare two models of those pathways; a neuron does not change its biological identity when you switch.
Click the centre or surround, or adjust the light with the sliders. Follow how the contributions and the cell's model response change.
1Where does the light fall?
2How do the contributions combine?
The model's relative contrast response
Equal illumination in both regions: the opposing contributions cancel in the balanced model, even when both are dark.
The same centre, with and without the surround contribution
Centre contribution alone
Centre + surround
The model: R_ON = (centre − surround) / 100 and R_OFF = −R_ON. It is balanced, so uniform light gives zero by construction — a property of the model, not a general physiological law. The model has no noise, time course or adaptation. The values are not measurements or firing rates.
Receptive fields and contrast enhancement
Centre–surround organisation contributes to contrast enhancement. Because light in the centre and light in the surround have opposing effects on the cell's response, differences between neighbouring parts of the visual field are emphasised. Imagine a light cup against a dark background. A receptive field near the cup's edge may receive more light in its centre than in its surround. The cell's response then provides information about a possible boundary between surfaces. It is the contrast in the nervous system's signals that is emphasised; the light from the cup does not change. Edges are one of several cues that work with colour, texture and context when we recognise an object.
When you look at the cup, each ganglion cell conveys information about part of the visual field. Many cells' receptive fields overlap, and different cell types process partly different properties in parallel. The pattern of activity across many cells therefore carries information both about where something happens in the visual field and about what changes there. An individual ganglion cell does not recognise the cup; its signals contribute to further processing.
The concept of a receptive field also applies to touch. Here it describes the area of skin that can influence a particular neuron's activity. The fields' size and their combined coverage of the skin help us understand how the nervous system can locate touch and distinguish details.
From the model to the retina
Photoreceptors influence ganglion cells through retinal circuits. Light hyperpolarises photoreceptors and reduces their glutamate release. Horizontal-cell feedback and feedforward influences help shape surround effects in the outer retina; amacrine circuits also contribute in the inner retina. This is not a simple chain in which neighbouring ganglion cells inhibit one another.
Centre–surround antagonism is a functional description. This subtraction is not a complete physiological mechanism, and not all ganglion cells have simple circular fields. It also does not by itself explain simultaneous contrast in section 2.1. Kuffler's classic recordings concern cats. Ströh and colleagues' mouse experiment shows that altered inputs to horizontal cells can change the surround of transient OFF-alpha ganglion cells — not that a single circuit explains every human receptive field.
Sources for the model and retinal explanation
- Kuffler, S. W. (1953). Discharge patterns and functional organization of mammalian retina. Journal of Neurophysiology, 16(1), 37–68. DOI: 10.1152/jn.1953.16.1.37.
Classic physiological recordings from cat ganglion cells: antagonistic centre/surround regions and variable responses. Not evidence for the model's exact balanced subtraction or all human ganglion cells.
- Ströh, S. et al. (2018). Eliminating Glutamatergic Input onto Horizontal Cells Changes the Dynamic Range and Receptive Field Organization of Mouse Retinal Ganglion Cells. Journal of Neuroscience. DOI: 10.1523/JNEUROSCI.0141-17.2018.
Primary mouse experiment: eliminating glutamate receptors on horizontal cells altered surround structure in transient OFF-alpha ganglion cells. Supports horizontal-circuit contributions alongside amacrine circuits, not a universal human circuit.
- Masland, R. H. (2012). The Neuronal Organization of the Retina.
The diversity and organisation of the cells; the tissue sketch shows only a selection.
- Purves, D. et al. (2001). Neuroscience (2nd ed.). The Retina.
The retina's layers, cell types and the direction of light in the visual lab.
- Gauthier, J. L. et al. (2009). Receptive Fields in Primate Retina Are Coordinated to Sample Visual Space More Uniformly. PLoS Biology, 7(4), e1000063.
Recordings in isolated peripheral macaque retina, from ON/OFF midget and parasol populations. Supports collective spatial sampling and overlapping receptive fields, not a direct demonstration of human object recognition or the lab's exact subtraction.
- Johansson, R. S. & Vallbo, A. B. (1980). Spatial properties of the population of mechanoreceptive units in the glabrous skin of the human hand. Brain Research, 184(2), 353–366.
Recordings of afferents from human glabrous hand skin and a population model. Supports the relationship of field size, density and coverage to tactile spatial information, not retinal centre–surround wiring in all skin afferents.
Illustrations and models for learning; not measurements.