Final answer:
The earliest attention effect observed in the visual cortex is the early selection theory, which suggests that attention acts as a filter at an early stage of sensory processing. This is demonstrated by experiments where subjects adapt to inverted visual stimuli over time.
Step-by-step explanation:
In the visual cortex, the earliest attention effect that is observed is known as the early selection theory. This theory proposes that attention acts as a filter that selectively processes information at an early stage of sensory processing. For example, in the experiment with prism glasses where subjects' visual field was inverted, the subjects initially reacted based on the inverted perception. However, over time, their visual cortex adapted and they behaved as if everything was represented correctly. This experiment demonstrates the flexibility of the visual cortex in adapting to the information it receives from our eyes and highlights the role of attention in shaping our perception.