The results suggest that the angle of difference between adapt and
test stimuli where the illusion is optimum is between 5-15º (clockwise or anticlockwise). The angle of difference at which the illusion stops working is when the
adapt stimulus tilted 60º or more from the test stimulus. There was also no effect when the
adapt and test stimuli were the same (vertical gratings).
The experiment provides strong evidence for the occurrence of the
tilt after-effect, with results demonstrating that prolonged exposure to a
tilted stimuli causes a perception of tilt in a subsequent vertical stimulus.
This supports the notion of adaptation as proposed by Movshon & Lennie (1979),
with cortical cells becoming fatigued and therefore less responsive in the
immediate period. The results support previous research suggesting that there
are optimal angles of difference for the occurrence of the effect. This seems
to concur with the 5-20 degree limits proposed by Howard and Templeton (1966).
The tilt after-effect did not occur when the angle of difference
exceeded 40º in either direction.
This is greater than angles proposed by Campbell and
Maffei (1971) and has important implications for understanding the range of
reactivity of individual cortical cells. The fact that the effect was apparent
for adapt stimuli tilted 40º from
vertical suggests that whilst an individual cortical
cell is maximally responsive to lines of a particular orientation, it continues
to respond to lines tilted up to 20º
away from this preferred orientation.
The experiment was not without limitation and
there were several anomalous results that suggested the tilt after-effect did
not occur for all participants. There are several possible reasons for this.
Firstly, the experiment was not conducted in neutral surroundings; there were
distractions, such as lighting, noise and other individuals in close proximity.
This undoubtedly affected the attention of participants and therefore it was
possible that their full attention was not given to the adapt stimulus for the
one-minute period. Also, several participants reported high levels of fatigue
during the experiment and reports of eye strain and headaches were recorded.
There is therefore the distinct possibility that this type of fatigue could
have adversely affected the results towards the end of the trials. In future it
would be appropriate to provide much greater rest periods between adapt stimuli
in order allow participants to recover from any undesirable side-effects such
as eye strain. These ‘rest’ periods may also facilitate increased attention to
the task on the part of participants.
To conclude, findings suggest evidence for the
tilt-after effect, supporting the theory of adaptation
and fatigue in line-detecting cortical cells of the visual system. Optimal
conditions for the occurrence of this effect have been suggested; with best
results obtained for adapt and test stimuli tilted 5-15º apart. Future
research is needed, with attention paid to reducing the effects of participant
(This reference section is not
meant to match exactly the references in the discussion – they are separate
examples, with the reference section containing additional examples to those
found in the discussion)
Campbell, F.W., & Maffei, L. (1971). The Tilt
After-effect: A fresh look. Vision Research, 11, 833-840.
Gibson, J.J., & Radner, M.
after-effect and contrast in the perception of tilted lines. Journal of
Experimental Psychology, 20, 453-467.
Greenlee, S., & Magnussen, M.W. (1987). Saturation
of the Tilt After-effect. Vision Research, 26(4), 661-672.
Harris, J.P., & Calvert, J.E. (1989). Contrast,
spatial frequency and test duration effects on the tilt after-effect:
implications for underlying mechanisms. Vision Research, 27(1),
Howard, I.P., & Templeton, W.B. (1966). Human
Spatial Orientation. London: Wiley.
Maffei, L., & Fiorentini, A. (1973). The visual
cortex as a spatial frequency analyser. Vision Research, 13,
Movshon, J.A., & Lennie, P. (1979).
Pattern-selective adaptation in visual cortical neurones. Nature, 278, 850–852.
Sekuler, R., & Blake, R. (2002). Perception, (4th Ed). New
York: McGraw Hill.