Biomedical subjects

J D Krol

Publications and source records attributed to J D Krol.

5 recordsLinked to original sources

PubMed1986

Binocular depth mixture: an artefact of eye vergence?

Several studies by Foley et al. suggest that for particular stereoscopic stimulus conditions perceived depth depends on the luminance ratios of the stimuli. Perceived depth appears "shifted" in the direction of the most luminant stimulus. This finding has been interpreted as contradictory evidence against modern neuronal network or algorithmic models which are extensions or variants of so-called projection theories of stereopsis. The present study shows that this interpretation lacks a solid basis and that the apparent depth shift effect probably qualifies as an artefact caused by systematic fixation errors (fixation disparities). With a psychophysical methodology which aims at preventing the occurrence of systematic fixation errors no depth shift effect is found.

Adult↗

PubMed1984

Exorcising the double-nail confusion: a reply to Ono.

Contrary to the claims of Ono it is argued that the principles and rules of direction vision do not account for the double-nail illusions, and that the 'ghost' concept is hidden rather than exorcised in Ono's approach.

Brain↗

PubMed1983

Depth from dichoptic edges depends on vergence tuning.

Small but reproducible fixation disparities occur in normal subjects when they view certain types of dichoptic stimuli. During dichoptic as well as stereoptic stimulation the motor fusion process determines first the average vergence state of the eyes. The subsequent fine tuning of vergence is shown to depend on the spatial distribution of contrast edges both of the same contrast sign ('stereoptic edges') and of opposite contrast sign ('dichoptic edges'). Stereoptic edges tend to induce superposition attempts of the vergence control system and dichoptic edges tend to antagonise this process. If a single low-contrast dichoptic edge is presented with zero disparity and within a stereoptic reference frame, a fixation disparity of several minutes of arc results. This influences depth vision since dichoptic edges are perceived (as monocular edges) at the actual rather than at the intended fixation distance. The findings explain previous paradoxical results of eg Kaufman and Pitblado who reported seeing depth in opposite-contrast stereograms. Their results seemed to contradict the well-established 'same-sign rule' (SSR) which states that the stereoptic system only detects disparities of edges with the same contrast sign. It is concluded that (i) the SSR holds; (ii) dichoptic (and monocular) edges are seen at the horopter; (iii) the vergence fine tuning prevents superposition of dichoptic edges even if this causes a fixation disparity.

Depth Perception↗

PubMed1982

Rehabilitation of a classical notion of Panum's fusional area.

It is argued that (1) the work of Burt and Julesz does not challenge the classical notion of Panum's fusional area; (2) their disparity gradient concept is contradicted by some of their own findings as well as by those of Krol and van de Grind; (3) their results can be accounted for by the classical concept of Panum's fusional area if eye vergence is taken into account; (4) the explanations suggested by Burt and Julesz for the depth perception aspects of Panum's limiting case and the double-nail illusion are based on unwarranted generalisation of results concerning binocular direction vision.

Depth Perception↗

PubMed1980

The double-nail illusion: experiments on binocular vision with nails, needles, and pins.

When two nails or similar slender objects are held straight ahead at reading distance, one a few centimetres behind the other and aligned at the same eye level, they are seen side by side rather than one behind the other. A quantitative study of this 'double-nail' illusion shows that the objects are judged to be at the positions of the so-called apparent or ghost images known from fusional theories of stereopsis. Most recent fusional theories assume that apparent images are suppressed by neuronal interactions, and the usual absence of percepts corresponding with apparent images is often quoted as an argument against 'projection' theories of stereopsis. The double-nail illusion shows, however, that percepts which correspond with apparent images do occur. The results are interpreted in terms of a neuronal-network type of fusional theory, in which the interpretation that corresponds with the minimum overall disparity is assumed to dominate. The following parameters were varied in the experiments: length, width, colour, and contrast for each of the nails; and fixation point position; and the orientation of the nail carrier. The results show that identity of the stimuli is not a necessary condition for the illusion. A stable vergence can be enforced by the double-nail illusion, and with additional nails multistable states of vergence can be obtained.

Color↗