Oblique Effect and Horizontal Effect
Horizontal Effect
If someone has their vision tested with stripes (right, top row) they typically see the horizontal and vertical stripes a little better than the oblique
stripes. However, if people look at noise (middle row) or a natural scene (bottom row), it is the oblique content that is best seen. Why is this? Perhaps one hint is that we (and other species) seem to have more neurons in visual cortex tuned to horizontal or vertical than to oblique orientations. Another hint is that in natural scenes, oblique content is least prevalent and horizontal and vertical content is most prevalent (with more horizontal than vertical content) (see figure below right). From recent work in our lab, we believe that we've been able to answer this question. Our answer is that the cortical neurons inhibit each other so having a greater number of neurons at the orientations that are most prevalent in the environment serves to turn down our sensitivity to the typical content in a scene and make other things relatively more salient in a scene. That is, the normal anisotropy in the visual world is discounted by this bias in the neural hardware, streamlining the encoding process, and relatively enhancing novel content. (See below for references.)
Background
On a wide variety of tasks, humans perform more poorly when the test stimuli are oriented obliquely than when oriented at horizontal or vertical. This anisotropic performance has been termed the oblique effect. Some time ago (Essock, 1980), we showed that there are really two types of oblique effect: oblique effects are obtained on tasks of basic visual ability ("Class 1") and are also obtained on tasks that emphasize memory and recall of stimulus orientations ("Class 2"). The class 1 oblique effect is always fixed to retinal coordinates and has been related to a corresponding bias in the number of (striate cortex) neurons that are tuned to these orientations (fewest at oblique orientations). Class 2 oblique effects can change with head-tilt, body-tilt or other manipulations, showing that the encoding and recall of orientation can be modified to match other coordinate systems. A similar effect has been reported for the haptic system. (For reviews see Essock, 1980 and Essock et al 1997 pdf.) Recent work using a priming RT task suggests that the Class 2 oblique effect stems from additional switching required to identify one oblique from others. This work is also continuing in our lab.
When presented a natural or other broad-band scene, people see oblique content the best and they actually see horizontal content the worst, with vertical usually falling in between (see figure below) -- we've termed this anisotropy the horizontal effect to match the terminology for the 
oblique effect seen with simple test stimuli (e.g., stripes on an eyechart). We feel that this horizontal effect is the result of cortical suppression between channels with similar orientations and scales (spatial frequencies). We've investigated the breadth of this suppression and its timecourse (Essock, Haun and Kim, see modeling section.) We feel that the local suppression serves to aid in the segmentation of one region from another, or an object from the background, and also leads to striking illusions (see illusions section). We also note that the horizontal effect seems strongest when viewing stimuli that are most like natural scenes (Hansen and Essock, 2005 pdf) and suggest that the evolution of this bias in the visual cortex is closely coupled with the content typical of outdoor visual scenes.
References
Hansen, BC and Essock, EA (2006) Anisotropic local contrast normalization: The role of stimulus orientation and spatial frequency bandwidths in the oblique and horizontal effect perceptual anisotropies. Vision Research, 46, 4398-4415. pdf
Hansen, BC and Essock EA. (2005) Influence of scale and orientation on the visual perception of natural scenes. Visual Cognition, 12, 1199-1234. pdf
Hansen, BC and Essock, EA. (2004) Biases in human visual processing of orientation and the structural components of natural scenes, Journal of Vision, 4(12), 1044-1060. pdf
Hansen, BC, DeFord, JK, Sinai, MJ, and Essock, EA. (2003) Manipulation of the amplitude and phase spectra of broadband stimuli: Implications for human visual processing of natural scenes. Network: Computation in Neural Systems, 14, 501-526. pdf
Essock, EA, DeFord, JK, and Hansen, BH (2003) Oblique stimuli are seen best (not worst!) in naturalistic broadband stimuli: A horizontal effect. Vision Research, 43, 1329-1335. pdf
Essock, EA, Krebs, WK and Prather, JR. (1997). Superior sensitivity for tactile stimuli oriented proximal-distally on the finger: Implications of mixed Class 1 and Class 2 anisotropies. Journal of Experimental Psychology: Human Perception and Performance, 23, 515-527. pdf
Essock, EA (1982). Anisotropies of perceived contrast and detection speed. Vision Research, 22, 1185-1191.
Essock, EA and Siqueland, ER (1981). Discrimination of orientation by human infants. Perception, 10, 245-253.
Essock, EA (1980). The oblique effect of stimulus identification considered with respect to two classes of oblique effects. Perception, 9, 37-46.
