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Introduction Example

in the Angle of Tilt: Impact on the Functioning of the Tilt After-Effect



Research concerning common
perception theories has proven critical in developing our understanding of the
structure of the human visual cortex. One of the most striking characteristics
of cortical cells is that they are orientation selective (Secular & Blake,
2002), where each cortical cell is maximally responsive to lines of a
particular orientation. If a line tilts a little away from this ‘preferred’ orientation
then the cell’s response decreases. The preferred orientation varies from cell
to cell such that an ensemble of cells will contain those sensitive to all


If these cortical cells are
stimulated for a long period of time, they undergo ‘adaptation’ and become
momentarily fatigued as a result of the prolonged exposure. This means that the
cells are then less responsive for a few seconds (Movshon & Lennie, 1979).
It is not only the specific cells for the particular orientation that become
fatigued, but the cells of similar orientations also become adapted, although
it is to a lesser extent. This theory can be used to explain the tilt
after-effect, “a temporary change in the perceived orientation of lines”
(Sekuler & Blake, 2002). This effect occurs when adaptation to a tilted
line causes a subsequent vertical line to be perceived ‘incorrectly’ as tilted
(in the opposite direction to the adapt stimulus).


The first published
research on the tilt after-effect was by Gibson and Radner (1937). Since then,
research has suggested that the period of adaptation has a significant
influence on the magnitude of the effect, with longer adaptation durations
producing a greater effect (Harris and Calvert, 1989) until saturation is
reached after approximately one hour (Greenlee and Magnusson, 1987).


Research has also explored
the effect of the angle of difference between the adapt and test stimuli on the
size of the effect.
Maffei and Fiorentini (1973) and  Movshon and Lennie, (1979) suggest that as the angle of difference increases the effect becomes
less apparent. Some suggest that the optimum angle of difference lies somewhere
between 5-20 degrees (Howard and Templeton, 1966) whereas others have claimed
that the illusion works best at 30 degrees of tilt. Estimations of where the
illusion stops working are also varied with some suggesting that the effect
ceases between 25-50 degrees (Campbell and Maffei, 1971).


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<![endif]--><!--[if !vml]-->image<!--[endif]-->There is therefore some debate about the exact parameters within
which the tilt after-effect works. This experimental study will explore the
effect of varying the angle of difference between the adapt and test stimuli on
the occurrence of the illusion. Two questions will be addressed;
firstly, at what angle of difference (between the test and adapt stimuli) is
the illusion most apparent? and secondly, at what angle of difference does the
illusion stop working?


Note: it is more likely
that you will present a single hypothesis here