Sensitization by annular surrounds: sensitization and dark adaptation

Barrel cortical neurons and astrocytes coordinately respond to an increased whisker stimulus frequency

Background

Nerve cells program the brain codes to manage well-organized cognitions and behaviors. It remains unclear how a population of neurons and astrocytes work coordinately to encode their spatial and temporal activity patterns in response to frequency and intensity signals from sensory inputs.

Results

With two-photon imaging and electrophysiology to record cellular functions in the barrel cortex in vivo, we analyzed the activity patterns of neurons and astrocytes in response to whisker stimuli with increasing frequency, an environmental stimulus pattern that rodents experience in the accelerated motion. Compared to the resting state, whisker stimulation caused barrel neurons and astrocytes to be activated more synchronously. An increased stimulus frequency up-regulated the activity strength of neurons and astrocytes as well as coordinated their interaction. The coordination among the barrel neurons and astrocytes was fulfilled by increasing their functional connections.

Conclusions

Our study reveals that the nerve cells in the barrel cortex encode frequency messages in whisker tactile inputs through setting their activity coordination.

Neuropsychological inference with an interactive brain: A critique of the “locality” assumption

When cognitive neuropsychologists make inferences about the functional architecture of the normal mind from selective cognitive impairments they generally assume that the effects of brain damage are local, that is, that the nondamaged components of the architecture continue to function as they did before the damage. This assumption follows from the view that the components of the functional architecture are modular, in the sense of being informationally encapsulated. In this target article it is argued that this “locality” assumption is probably not correct in general. Inferences about the functional architecture can nevertheless be made from neuropsychological data with an alternative set of assumptions, according to which human information processing is graded, distributed, and interactive. These claims are supported by three examples of neuropsychological dissociations and a comparison of the inferences obtained from these impairments with and without the locality assumption. The three dissociations are: selective impairments in knowledge of living things, disengagment of visual attention, and overt face recognition. In all three cases, the neuropsychological phenomena lead to more plausible inferences about the normal functional architecture when the locality assumption is abandoned. Also discussed are the relations between the locality assumption in neuropsychology and broader issues, including Fodor's modularity hypothesis and the choice between top-down and bottom-up research approaches.

The role of spatial filtering in sensitivity regulation

The role of spatial filtering in controlling sensitivity to increments is hard to evaluate under normal viewing conditions because eye movements lead to a confounding of spatial and temporal transients. We measured sensitivity to increments on different sized backgrounds in photopic and scotopic vision when the backgrounds were stabilized on the retina, thus eliminating temporal transients. The saturating effect of small fields on photopic thresholds was preserved under these conditions indicating that spatial filtering by retinal cells is critical in maintaining photopic sensitivity. Some effect of spatial pattern on sensitivity in stabilized vision was also observed in scotopic vision, although it was much smaller than was observed in photopic vision. The interaction effects between rod and cone systems that are observed with small backgrounds were also preserved in stabilized vision, implicating a very peripheral site for the generation of these interactions.

Cone-rod interaction over time and space

Scotopic background stimulation can elevate photopic increment thresholds by more than 2 log units. This cone-rod interaction is greatest on small backgrounds (less than 1 degree diameter), but is found consistently on large backgrounds as well. Interaction develops and disappears quickly as backgrounds are turned on or off, respectively. The onset, and in some cases the offset, of a background stimulus can produce an additional, transitory interaction that augments the interaction that is maintained by continued presentation of the same background. The majority of the present findings lend support to a simple center-surround model of cone-rod interaction: nearby scotopic excitation raises photopic thresholds and more distant scotopic stimulation primarily antagonizes this interaction.

The cone threshold: spatial interactions of rod and cone adapting signals

Steady small blue adapting fields that stimulated only rods produced large threshold elevations for a superimposed tiny red flash that stimulated cones. The threshold elevation differed considerably between observers, confirming the results of Buck (Topical Meeting on Recent Advances in Vision, abstract in Technical Digest, 1980; Invest. Ophthal, visual Sci., Suppl. 20, 207, 1981). A red annulus that stimulated cones reduced slightly the threshold elevation produced by the small rod adapting field. Similarly, when the cone threshold was elevated by a small red field that stimulated only cones, a blue annulus that stimulated rods slightly reduced the threshold of the red cone test flash. These effects, although weak, demonstrate lateral sensitizing interactions between a cone center field and rod annulus, and vice versa, when the cone threshold is assessed with a tiny red flash.

Lateral interactions in the control of visual sensitivity

Threshold vs intensity curves for cone vision, measured in the parafoveal retina, quickly saturate if the adapting background is made small (e.g. 19' at 5 degrees eccentricity). Log increment threshold increases at a rate of about 3:1 with log background illuminance at levels as low as 10 td. This shows that lateral interactions are an important process in preserving differential sensitivity in cone vision across the wide range of illuminances over which it normally operates. Parallels between light and dark adaptation in the effect of field size were explored, since effects of comparable magnitude are observed in both. Backgrounds and bleaches equated for their effects at one field size do not have equal effects on threshold at other field sizes, however, with small-area bleaches raising threshold more than predicted. This failure of equivalence was also revealed in a second experiment, in which recovery of sensitivity following small area bleaches was measured in the presence of large steady background fields, which have the effect of lowering threshold. Thresholds following the small bleach were lowered less than expected on the basis of the "equivalent background" hypothesis, a result which we take to mean that signals from bleached cones exceed those produced by a background which has an equivalent effect on threshold (the "equivalent background"). Control experiments examined whether rods contribute to the overloading of cone response by small fields and the possible contribution of such central adaptation processes as spatial frequency adaptation.

Rod-cone interaction in the dark-adapted fovea

The foveal cone threshold was significantly lower after 45 min of dark adaptation than it was near the start of the cone plateau of the dark-adaptation curve. A concentric rod background subsequently raised the threshold by an amount correlated wit the difference between the cone plateau and the dark-adapted thresholds. Paradoxically, the rod background also lowered the cone threshold by an amount that differed from subject to subject. This sensitizing effect was identifiable by its relatively small variability across sessions. These results show that adaptation of parafoveal rods by either real light or dark light can change foveal cone thresholds.

Background configuration and rod threshold

1. This paper investigates the variation in rod threshold when a small test flash is seen against backgrounds of different sizes. Over a substantial range of luminances above absolute threshold, the test flash is less easily seen against small backgrounds than large. This confirms earlier results.2. If an annular surround is added to a small circular background, threshold is reduced when background and annulus are equiluminous (uniform field), but rises rapidly as the annulus is made brighter or dimmer than the background. This cannot be explained by the threshold-elevating effects of light scattered on to the background from the surround, for threshold rises with annulus luminance faster than it does on uniform fields of equal luminance.3. If the surround is not a complete annulus but a windmill-shaped cross, threshold is higher than on a uniform field, no matter what the windmill luminance. Thus it is not the addition of light per se to the surround which reduces threshold.4. This conclusion is reinforced by the results of another experiment. The test flash is seen on a large uniform field. When superimposed on this field, a thin ring, light or dark, which causes only a small change in mean luminance, produces an appreciable rise in threshold.5. The addition of an equiluminous red surround to a small red background so as to create a uniform field causes a marked drop in test flash threshold, but a scotopically equal blue surround, that creates a uniform field for rods, does not alter the threshold. Since the test flash is seen only by rods it follows that signals from cones can alter rod threshold.6. Known or probable behaviour of retinal mechanisms cannot account for our results. All the operations which elevate threshold above its level on a large uniform field produce contours in the vicinity of the test flash. This we take as evidence that signals from the test stimulus are suppressed or reduced by other signals present only when the background is locally non-uniform.