On 2016 Jan 06, Lydia Maniatis commented:

This article finally clarified what the authors of this and similar papers mean by “biologically-generated” or “evolved” stimuli. Here are the relevant statements:

The visual system achieves its goals on the basis of “the creation of light energy patterns whose frequency of occurrence tracks reproductive success...these patterns are “stimuli” ...Photon energy in the environment causes receptor cells in the retina to respond...It is only after the evolved apparatus of the eye collects, focuses, and selects a particular fraction of this energy into a biologically determined pattern that a stimulus exists...light stimuli are determined by a collaboration between physics and evolved biology.”

Thus, Purves et al “stimulus” is simply the physiological state of the retina, which is contingent on incoming light that is focussed. I don't think anyone would argue with that. It acknowledges that the “stimulus” is connected with the physics of the world around us, specifically, the light impinging on the retina.

However, this description of the visual stimulus does not in any way justify or explain the notion that the “frequency of occurrence” of these stimuli "tracks reproductive success.” The pattern of retinal stimulation changes with each movement of the eye, and the patterns, depending as they do on the chance arrangements of the environment and on the line of sight, on the time of day, etc, could fairly be described as “random.” Even if we could somehow define, categorise and track particular “patterns,” why should their frequency of occurrence track reproductive success? Also, since the slate, insofar as the frequency of occurrence of these light patterns is concerned, is wiped clean with every generation, how are these frequency distributions preserved?

The repeated claims that the visual system can't use the stimulus to infer features of the physical world is contrdicted by the authors, who say that “the visual system does not take the measure of physical reality...that we readily assess with the instruments of physics.” In fact, we readily assess using our perception of the physical properties of the instruments of physics. In addition, the fact that focussing of light achieves a point to point correspondence between points on the retina and points in the world that reflected the light is suggestive of an attempt to infer the shapes and locations of actual physical objects.

The claims about the inability of the v.s. to access properties of the physical world is made based on our perception of pictorial mimics of projections from real-world scenes. If I compare my perceptions of the color of objects in a naturally-lit real-life scene under inhomogeneous illumination, then my perception of the intrinsic surface properties of objects (e.g. reflectance, chromaticity) will appear quite reliable. If I then take a photo of that scene, and compare my perceptions of its surface properties to the actual properties of the photo surface, then, of course, errors all around. This doesn't justify denying the veridical outcome in the natural scene, or discounting it in accounts of how perception works.

The methods are oddly weak. "Three hundred and eight high-resolution natural red–green–blue (RGB) images from The University of Texas at Austin “set 1” database (7) served as a proxy for human spectral experience.” [The sampling problem should speak for itself. The sample was collected by a different team with different priorities. We are not given a description of the images, or of the rationale with which they were collected. The lighting conditions are not controlled for before or after the fact. Perceptual effects are not tested. The claim that it is a proxy for human evolutionary experience is to be taken on faith. The argument of this paper hinges on this database.]

On 2016 Jan 06, Lydia Maniatis commented:

This article finally clarified what the authors of this and similar papers mean by “biologically-generated” or “evolved” stimuli. Here are the relevant statements:

The visual system achieves its goals on the basis of “the creation of light energy patterns whose frequency of occurrence tracks reproductive success...these patterns are “stimuli” ...Photon energy in the environment causes receptor cells in the retina to respond...It is only after the evolved apparatus of the eye collects, focuses, and selects a particular fraction of this energy into a biologically determined pattern that a stimulus exists...light stimuli are determined by a collaboration between physics and evolved biology.”

Thus, Purves et al “stimulus” is simply the physiological state of the retina, which is contingent on incoming light that is focussed. I don't think anyone would argue with that. It acknowledges that the “stimulus” is connected with the physics of the world around us, specifically, the light impinging on the retina.

However, this description of the visual stimulus does not in any way justify or explain the notion that the “frequency of occurrence” of these stimuli "tracks reproductive success.” The pattern of retinal stimulation changes with each movement of the eye, and the patterns, depending as they do on the chance arrangements of the environment and on the line of sight, on the time of day, etc, could fairly be described as “random.” Even if we could somehow define, categorise and track particular “patterns,” why should their frequency of occurrence track reproductive success? Also, since the slate, insofar as the frequency of occurrence of these light patterns is concerned, is wiped clean with every generation, how are these frequency distributions preserved?

The repeated claims that the visual system can't use the stimulus to infer features of the physical world is contrdicted by the authors, who say that “the visual system does not take the measure of physical reality...that we readily assess with the instruments of physics.” In fact, we readily assess using our perception of the physical properties of the instruments of physics. In addition, the fact that focussing of light achieves a point to point correspondence between points on the retina and points in the world that reflected the light is suggestive of an attempt to infer the shapes and locations of actual physical objects.

The claims about the inability of the v.s. to access properties of the physical world is made based on our perception of pictorial mimics of projections from real-world scenes. If I compare my perceptions of the color of objects in a naturally-lit real-life scene under inhomogeneous illumination, then my perception of the intrinsic surface properties of objects (e.g. reflectance, chromaticity) will appear quite reliable. If I then take a photo of that scene, and compare my perceptions of its surface properties to the actual properties of the photo surface, then, of course, errors all around. This doesn't justify denying the veridical outcome in the natural scene, or discounting it in accounts of how perception works.

The methods are oddly weak. "Three hundred and eight high-resolution natural red–green–blue (RGB) images from The University of Texas at Austin “set 1” database (7) served as a proxy for human spectral experience.” [The sampling problem should speak for itself. The sample was collected by a different team with different priorities. We are not given a description of the images, or of the rationale with which they were collected. The lighting conditions are not controlled for before or after the fact. Perceptual effects are not tested. The claim that it is a proxy for human evolutionary experience is to be taken on faith. The argument of this paper hinges on this database.]