In psychology, emotional reactions to stimuli are called affective responses. Affective responses happen very fast, and are governed in an automatic, unconscious way by the lower centers of the brain that also govern basic instincts (food, fear, sex, breathing, blinking, etc.). Think of affective responses as the brain’s bottom-up reaction to what you see and feel. Cognitive responses are your brain’s slower, top-down, more considered responses. They’re governed by your personal cultural views, learning, experiences, and personal preferences that you are aware of and can easily articulate. Affective reactions assign value to your experiences; cognitive reactions assign meaning to what you see and use.
He even offers a measurable application of this viewpoint:
Research confirms that users make aesthetic decisions about the overall visual impression of web pages in as little as 50 milliseconds (1/20th of a second). These instant visceral reactions to web pages happen in virtually all users, are consistent over visit length, and strongly influence the user’s sense of trust in the information. In short, users have made fundamental, consistent, and lasting aesthetic decisions about the credibility and authority of sites before major eyetracking events begin.
While I think any designer worth his salt instinctually knows this, there’s an important truth to consider — any site with an objective to establish trust in users and confidence in doing business should execute on multiple levels — and at the visceral level it happens nearly instantaneously with users.